Chop the onion and fry it for 15 minutes at low/medium heat. Be careful so the onion does not turn brown.
After frying for 10 minutes add the garlic using a garlic press (or finely chopped)
Add the canned tomatoes, vinegar, sugar, basilic, salt and pepper
Simmer for approx 15 minutes while steering occasionally
Bechamel sauce
Melt the butter and add the flour
Let it simmer at medium heat for a minute while you stir continuisly
Add the milk in several small portions, whisking vigorously in between each dose. The mixture needs to become smooth without lumps with every dose of milk you add to the mixture
Let it simmer for 10 minutes while stirring to avoid it to burn
Add salt, pepper and grated nutmeg to taste
Completing the Lasagna
Heat the oven to 220°c
Cut the raw salmon in pieces of aprox 2x2 cm and add them to the finished tomato sauce (without cooking them!)
Butter an ovenproof dish and start by a layer of 1/3 of the tomato/salmon sauce, cover with pasta sheets, add a thin layer of bechamel and add a bit of grated parmesan
Repeat the above two more times
Drissle the grated cheese on top and bake for 20 minutes
Ideally, you should let the lasagna rest for 10-15 minutes before to cut it