While your quinoa cooks (if not already prepared), make both sauces to maximize efficiency. For the vinaigrette, whisk together the red wine vinegar, olive oil, mustard, honey, dried oregano, and 1/4 tsp salt in a small bowl—this will be your marinade and dressing. In a separate bowl, combine the yogurt, lemon juice, lemon zest, fresh dill, minced garlic, and remaining 1/4 tsp salt and pepper to create your tzatziki sauce. Set both aside. I like to make the tzatziki at least 15 minutes ahead so the flavors have time to meld together.

Pat the beef cubes dry with paper towels, then toss them in a bowl with 2 tablespoons of the vinaigrette from Step 1 and the 2 tsp Greek seasoning. Let sit for at least 15 minutes (up to 2 hours for deeper flavor). Heat a cast-iron skillet or heavy-bottomed pan over medium-high heat until it reaches 375-400°F. Working in batches to avoid crowding, sear the beef cubes for 10-12 minutes, turning occasionally and basting with the remaining vinaigrette during the last 5 minutes to build a flavorful glaze. The beef should reach an internal temperature of 145-160°F depending on your doneness preference. Remove the seared beef to a plate and let rest for 3 minutes before adding to bowls.

While the beef sears, bring a large pot of salted water to a boil and add the spinach, stirring until it wilts and turns bright green (about 2-3 minutes). Drain well in a colander, pressing gently to remove excess water. Dice the tomatoes into 1/2-inch pieces and the cucumber into similar-sized pieces. I find that cutting both vegetables at the same size helps them distribute evenly throughout the bowl and look more polished.

Divide the cooked quinoa from Step 1 (or prepared quinoa) evenly among serving bowls as your base. Top each bowl with a portion of the steamed spinach from Step 3, then arrange the diced tomatoes and cucumber around the bowl. Add the seared beef from Step 2 to the center of each bowl, then drizzle generously with the tzatziki sauce from Step 1. Serve immediately while the beef is still warm.