Prepare the leek fondue:

Trim about 2 inches off the green tops of the leeks. Wash them thoroughly and slice them finely.

In a sauté pan, melt 1.75 tablespoons (about 25 g) of butter over medium heat.

Add the sliced leeks and a pinch of salt. Cook gently without browning, reducing heat if needed.

Once soft, stir in the crème fraîche and mustard. Set aside.

Make the béchamel sauce:

In a saucepan, melt the remaining 2.75 tablespoons of butter (about 40 g).

Add the flour and cook for 1 minute, stirring constantly.

Gradually pour in the cold milk, whisking continuously until the sauce thickens.

Season with salt, pepper, and a pinch of nutmeg.

Prep the salmon:

Cut the salmon into 1/2-inch (1 cm) cubes.

Assemble the lasagna (using a 5.5 x 7 inch baking dish or similar):

• Spread a thin layer of béchamel sauce on the bottom.
• Add a layer of lasagna noodles.
• Spoon over some of the leek mixture, then scatter some salmon cubes, and cover with béchamel.
• Repeat the layers until the dish is full.
• Finish with a final layer of noodles, a generous coating of béchamel, and sprinkle with grated Parmesan.

Bake:

Preheat the oven to 350°F (180°C).

Bake for 30 to 40 minutes, until golden and bubbling.