Preheat the oven to 200º C.
In a pan, sauté the chopped onion and tomato in oil. Add the tomato pulp, the tuna and chopped parsley. Season with pepper, salt and drizzle with white wine. Let cook for 3 minutes over medium heat, stirring constantly.
For bechamel sauce: melt butter in a saucepan and then add the flour, stirring well, until a lump-free mixture is formed. Add the milk little by little, stirring constantly and let it cook over a low heat until it thickens. Season with the remaining salt and set aside (you can use the béchamel sauce in a package that you buy at the supermarket).
Lay in a square oven tray: the lasagna sheets, the tuna preparation and the bechamel sauce. Repeat the process, ending with a layer of lasagna sheets.
Sprinkle with mozzarella cheese and bake at 200º C for 40 minutes.