Cut the courgettes and the onion into thin slices. Melt a knob of butter in a pan, add the courgettes and onions, a pinch of salt, pepper and toss for about 10 minutes.
Cooking must be quick.
Cut the mozzarella into cubes and set aside.
Heat a saucepan with water and when it bubbles add a teaspoon of oil and a handful of salt. Blanch the pasta dipping one sheet at a time for about 1 minute, then take it with a skimmer and lay it on a cloth towel so that it dries. Do this for all the sheets.
Cut the salmon into thin slices and begin the assembly. Butter the surface of the lasagna dish and add ingredients as follows: two scoops of white sauce, a layer of sheets of pasta, courgettes, salmon, mozzarella and Parmesan cheese to cover the entire surface. Continue this sequence for three more layers to the last layer.
Then, add the butter flakes to brown the outer surface of the lasagna.
Put the dish in the preheated oven and bake at 180 degrees for about 40 minutes.