1. Preheat your oven to 180°C (350°F).
2. Using a suitable grater, grate the gruyère cheese and nutmeg separately in two separate bowls.
3. Set them aside.
4. Cut the lemon in half and squeeze the juice from one half into a small container.
5. On a cutting board, cut your salmon fillets into 4 pieces.
6. Place a saucepan over medium heat.
7. Place the salmon pieces in the saucepan and cover with cold water.
8. Add salt and lemon juice.
9. Cook for 6 minutes.
10. Once the salmon pieces are cooked, drain them in a colander.
11. Let them cool.
12. Then arrange them on a plate and crumble them with a fork.
13. Wash your leeks and separate the white parts from the green parts.
14. Slice the white parts of the leeks.
15. Place 30g of butter, a little water, and salt in a saucepan over high heat and heat it up.
16. Cook the white parts of the leeks covered for 10 minutes.
17. Once cooked, remove the white parts of the leeks and drain them.
18. In a bowl, mix the white parts of the leeks with the cream and a little salt.
19. Place 30g of butter in a plate and melt it in the microwave.
20. In another saucepan, prepare your béchamel sauce by mixing the melted butter, flour, cold milk, salt, and a little grated nutmeg.
21. Place this saucepan over heat and bring the mixture to a boil while whisking for 2 minutes.
22. Take a gratin dish and grease it.
23. Layer the lasagna sheets, leeks, salmon, and cover with béchamel sauce.
24. Finish by sprinkling grated gruyère cheese on top.
25. Bake for 30 minutes.