Preheat your oven to 400°F (200°C). While it heats, prepare all ingredients: trim the asparagus and cut into 2-inch pieces, keeping the tips separate from the thicker stems—this separation matters because stems need more cooking time. Cut the salmon into 1-inch chunks and refrigerate until ready to use. Finely slice the shallots and measure out your milk, cream, flour, and cheese. This prep work ensures smooth cooking once you begin.

In a small bowl, combine the panko, grated parmesan cheese, and chopped parsley. Drizzle the melted butter over this mixture and toss with a fork until the panko is evenly coated and slightly damp—this helps it brown beautifully in the oven. Set aside until ready to top the gratin.

Heat the 1 oz butter and olive oil together in a large skillet or saucepan over medium heat. Once the butter foams, add the sliced shallots and sauté for 3 minutes until softened and fragrant. Add the asparagus stems and cook for 2-3 minutes to start softening them. Sprinkle the flour over everything and stir constantly for 1 minute to create a roux base—this prevents lumps in your sauce. Gradually pour in the milk and cream while stirring continuously until the mixture thickens, about 2-3 minutes. I like to add a pinch of ground nutmeg at this point because it subtly enhances the cream without overpowering the salmon and asparagus.

Remove the skillet from heat and stir in the shredded cheddar cheese until completely melted and smooth. Season with salt to taste. Gently fold in the asparagus tips (which only need brief cooking), then add the salmon chunks and stir carefully to coat everything evenly with the creamy sauce—be gentle with the salmon to avoid breaking it into smaller pieces. Transfer the entire mixture to a buttered 9x13-inch baking dish (or similar size) and spread it into an even layer.

Sprinkle the panko-parmesan topping from Step 2 evenly over the gratin, distributing it from edge to edge for even browning. Place in the preheated 400°F oven and bake for 15 minutes until the topping is golden brown and the sauce is bubbling gently around the edges. I find that the panko topping gives a much lighter, crunchier texture compared to regular breadcrumbs, and the higher heat helps it crisp beautifully while the salmon stays moist underneath. The gratin is done when you see golden color on top and can glimpse a gentle bubble or two at the edges.