Melt 2 tablespoons of unsalted butter in a large skillet over low heat. Add sliced leeks and sauté for 5–10 minutes until they soften and become translucent. Stir in the drained fresh or thawed frozen spinach, sprinkle with salt, and cook until just wilted. Drain any excess moisture using a sieve to prevent a watery lasagna.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 4 tablespoons of all-purpose flour to form a roux, cooking for about 2 minutes until bubbling. Gradually whisk in 2 cups of milk, stirring continuously for 5–7 minutes until the sauce thickens and coats the back of a spoon. Stir in Dijon mustard, dill, lemon zest, nutmeg (if using), and half of the grated Parmesan, seasoning to taste.

Preheat your oven to 350°F (180°C). In an ovenproof dish, spread a thin layer of béchamel sauce on the bottom, then layer the no-pre-cook lasagna sheets on top. Add a layer of flaked salmon, followed by a generous portion of the spinach filling, and then more béchamel. Repeat the layers, finishing with a top layer of béchamel sauce and the remaining grated Parmesan.

Cover the assembled Salmon Lasagna with aluminum foil and place it in the preheated oven. Bake for 25 minutes, or until the sauce is bubbling. For an extra golden top, remove the foil and broil on high for an additional 5 minutes, keeping a close eye to prevent burning.

Once out of the oven, allow the lasagna to cool for about 10 minutes for easier slicing and to let the flavors meld together. Cut into squares and serve with a side salad or steamed vegetables.