Easy Chicken Spinach Sandwich

I used to think sandwiches had to be boring—just some cold cuts slapped between bread with maybe a piece of lettuce if I was feeling fancy. That all changed when I started playing around with warm, seasoned chicken and quick-pickled onions.

The trick is in the layers. You’ve got tender chicken strips with a little kick from red pepper flakes, sweet-and-tangy pickled onions that take maybe five minutes to throw together, and fresh spinach for some green. Add melted provolone and a garlic mayo, and suddenly you’re eating something that feels like it came from a café—but you made it in your own kitchen on a regular Tuesday.

What Kind of Chicken Should I Use?

For this sandwich, boneless skinless chicken cutlets are your best bet since they cook quickly and evenly. If you can’t find pre-cut cutlets at the store, you can easily make your own by slicing a regular chicken breast in half horizontally to create two thinner pieces. The thinner cutlets are key here because they’ll cook through faster and fit better on your sandwich roll. If all you have are regular thick chicken breasts, just pound them out with a meat mallet to about half an inch thick, and you’ll be good to go.

Options for Substitutions

This sandwich is easy to customize based on what you have in your kitchen:

• Red onion: Yellow or white onions work just as well for the caramelized topping. They’ll taste slightly sweeter, but still give you that nice jammy texture.
• Apple cider vinegar: White wine vinegar, red wine vinegar, or even balsamic vinegar can step in here. Each will add a slightly different flavor, but all will give you that tangy kick.
• Chicken cutlets: Regular chicken breasts work fine – just pound them thin or slice them horizontally to create cutlets. You could also use leftover rotisserie chicken or even turkey cutlets.
• Provolone cheese: Swiss, mozzarella, or even cheddar would be tasty alternatives. Pick whatever melty cheese you like best.
• Baby spinach: Arugula, mixed greens, or regular lettuce all make good substitutes. If using regular spinach, just remove the thick stems first.
• Sandwich rolls: Ciabatta, kaiser rolls, or even sliced sourdough bread would work great. Just toast them lightly for the best texture.

Watch Out for These Mistakes While Cooking

The biggest mistake when making this sandwich is not pounding the chicken to an even thickness, which leads to dry edges while the center is still cooking – aim for about 1/2 inch thick throughout so everything cooks at the same rate.

Don’t rush the caramelized onions by cranking up the heat, as they need that low and slow cooking time to develop their sweet flavor without burning, and if they start to stick, add a splash of water to the pan.

Another common error is skipping the resting time after cooking the chicken – let it sit for 3-5 minutes before slicing so the juices redistribute instead of running all over your cutting board.

Finally, toast your buns lightly before assembling to prevent them from getting soggy from the mayo and spinach, and make sure your chicken reaches 165°F in the thickest part to ensure it’s fully cooked.

What to Serve With Chicken Spinach Sandwich?

A chicken spinach sandwich pairs perfectly with classic potato chips or sweet potato fries on the side – something crunchy to balance out the soft sandwich. If you want something lighter, a simple coleslaw or a crisp cucumber salad with a light vinaigrette works great and won’t weigh you down. You could also go with some roasted vegetables like zucchini or bell peppers, or even a cup of tomato soup for dipping if you’re extra hungry. For a quick weeknight meal, I usually just grab a bag of kettle chips and call it a day!

Storage Instructions

Store: This sandwich is best enjoyed fresh, but if you have leftovers, store the components separately. Keep the cooked chicken and caramelized onions in airtight containers in the fridge for up to 3 days. The spinach and cheese should also be stored separately to keep everything from getting soggy.

Make Ahead: You can definitely prep parts of this sandwich ahead of time. The caramelized onions actually taste even better the next day and can be made up to 5 days in advance. Cook the chicken up to 2 days ahead and store it in the fridge, then just reheat and assemble when you’re ready to eat.

Reheat: Warm up the chicken in a skillet over medium heat for a few minutes on each side, or microwave it for about 30 seconds. The caramelized onions can be warmed in the microwave or on the stovetop. Toast your rolls fresh and assemble everything right before serving for the best texture.

Estimated Nutrition

Estimated nutrition for the whole recipe (without optional ingredients):

• Calories: 800-950
• Protein: 48-56 g
• Fat: 28-36 g
• Carbohydrates: 74-86 g

Ingredients

For the caramelized onions:

• 1/2 red onion (thinly sliced into 1/8-inch half-moons)
• 1/2 tbsp olive oil
• 1 1/2 tbsp sugar
• 1 tbsp apple cider vinegar
• 1 pinch salt

For the garlic spread:

• 1/8 cup mayonnaise
• 1 1/2 tsp garlic (freshly minced)
• 1/4 tsp crushed red pepper flakes

For the sandwich assembly:

• 12 oz chicken (cut into 1/2-inch strips)
• 1/2 tsp italian seasoning
• 2 rolls (I recommend King’s Hawaiian sub rolls)
• 3/4 cup spinach
• 2 slices provolone

Step 1: Prepare the Caramelized Red Onions

• 1/2 red onion
• 1/2 tbsp olive oil
• 1 1/2 tbsp sugar
• 1 tbsp apple cider vinegar
• 1 pinch salt

Heat the olive oil in a skillet over medium heat.

Add the thinly sliced red onion half-moons and cook for 5 minutes, stirring occasionally until they begin to soften.

Add the sugar and apple cider vinegar, then reduce heat to low and cook for another 5-10 minutes, stirring frequently, until the onions are deeply caramelized and jammy.

Season with a pinch of salt.

Transfer to a bowl and set aside—these can stay at room temperature while you prepare the rest of the sandwich components.

Step 2: Make the Garlic Mayo and Season the Chicken

• 1/8 cup mayonnaise
• 1 1/2 tsp garlic
• 1/4 tsp crushed red pepper flakes
• 12 oz chicken
• 1/2 tsp italian seasoning

While the onions cook, combine the mayonnaise, freshly minced garlic, and crushed red pepper flakes in a small bowl and mix until smooth—set aside.

Pat the chicken strips dry with paper towels, then season them evenly with the Italian seasoning on both sides.

I like to let the seasoned chicken sit for a minute or two so the flavors can start to penetrate the meat.

Step 3: Cook the Chicken Until Golden

• seasoned chicken from Step 2

In the same skillet used for the onions (or a fresh one if needed), heat it over medium-high heat until shimmering.

Add the seasoned chicken strips and cook for about 5 minutes on the first side without moving them, allowing them to develop a golden-brown crust.

Flip and cook the other side for another 5 minutes until the chicken is cooked through and golden on both sides.

Transfer to a plate.

Step 4: Toast the Buns and Assemble the Sandwiches

• 2 rolls
• garlic mayo mixture from Step 2
• 3/4 cup spinach
• cooked chicken from Step 3
• caramelized red onions from Step 1
• 2 slices provolone

Preheat your oven to 375°F.

Place the King’s Hawaiian rolls or sub rolls open-faced on a baking sheet.

Spread the garlic mayo mixture from Step 2 generously on both halves of each roll.

On the bottom half of each roll, layer the spinach first, then the cooked chicken strips from Step 3, followed by the caramelized red onions from Step 1, and finally top with a slice of provolone cheese.

For the best result, I like to slightly overlap the cheese so it melts evenly across the sandwich.

Step 5: Bake and Finish

Bake the assembled sandwiches in the preheated 375°F oven for 5 minutes, or until the cheese is melted and the buns are warm and slightly toasted.

Remove from the oven, place the top halves of the rolls on each sandwich, and serve immediately while the cheese is still warm and gooey.

Easy Chicken Spinach Sandwich

Delicious Easy Chicken Spinach Sandwich recipe with step-by-step instructions.

Print Recipe Pin Recipe

Prep Time 20 minutes mins

Cook Time 35 minutes mins

Total Time 55 minutes mins

Servings 2 servings

Calories 875 kcal

Ingredients

  

For the caramelized onions

• 1/2 red onion (thinly sliced into 1/8-inch half-moons)
• 1/2 tbsp olive oil
• 1 1/2 tbsp sugar
• 1 tbsp apple cider vinegar
• 1 pinch salt

For the garlic spread

• 1/8 cup mayonnaise
• 1 1/2 tsp garlic (freshly minced)
• 1/4 tsp crushed red pepper flakes

For the sandwich assembly

• 12 oz chicken (cut into 1/2-inch strips)
• 1/2 tsp italian seasoning
• 2 rolls (I recommend King's Hawaiian sub rolls)
• 3/4 cup spinach
• 2 slices provolone

Instructions

 

• Heat the olive oil in a skillet over medium heat. Add the thinly sliced red onion half-moons and cook for 5 minutes, stirring occasionally until they begin to soften. Add the sugar and apple cider vinegar, then reduce heat to low and cook for another 5-10 minutes, stirring frequently, until the onions are deeply caramelized and jammy. Season with a pinch of salt. Transfer to a bowl and set aside—these can stay at room temperature while you prepare the rest of the sandwich components.
• While the onions cook, combine the mayonnaise, freshly minced garlic, and crushed red pepper flakes in a small bowl and mix until smooth—set aside. Pat the chicken strips dry with paper towels, then season them evenly with the Italian seasoning on both sides. I like to let the seasoned chicken sit for a minute or two so the flavors can start to penetrate the meat.
• In the same skillet used for the onions (or a fresh one if needed), heat it over medium-high heat until shimmering. Add the seasoned chicken strips and cook for about 5 minutes on the first side without moving them, allowing them to develop a golden-brown crust. Flip and cook the other side for another 5 minutes until the chicken is cooked through and golden on both sides. Transfer to a plate.
• Preheat your oven to 375°F. Place the King's Hawaiian rolls or sub rolls open-faced on a baking sheet. Spread the garlic mayo mixture from Step 2 generously on both halves of each roll. On the bottom half of each roll, layer the spinach first, then the cooked chicken strips from Step 3, followed by the caramelized red onions from Step 1, and finally top with a slice of provolone cheese. For the best result, I like to slightly overlap the cheese so it melts evenly across the sandwich.
• Bake the assembled sandwiches in the preheated 375°F oven for 5 minutes, or until the cheese is melted and the buns are warm and slightly toasted. Remove from the oven, place the top halves of the rolls on each sandwich, and serve immediately while the cheese is still warm and gooey.