Begin by heating plenty of water in a large pot. When it starts to boil, add salt and the lasagna sheets one by one, and cook following the packaging instructions.
Once cooked, drain and place on a cloth to remove excess water.
For the béchamel, melt the butter in a pot. Add the flour and move for 5 minutes to brown. Next, add the warm milk, salt, nutmeg and pepper. Stir continuously until obtaining a creamy texture. Set aside.
While the pasta is boiling, prepare the tuna filling. In a bowl, chop the eggs and tuna and add the fried tomato to taste. Mix.
Assemble the lasagna. In an oven dish, place sheets of lasagna on the bottom, covering them with the tuna filling, adding béchamel and 3 slices of cheese. Repeat the process (pasta, filling, béchamel and cheese) twice more.
Add a final layer of pasta and cover the lasagna with béchamel and grated cheese to taste.
Preheat the oven to 350ºF. Bake until the cheese melts. Remove from the oven and serve.