Cook the pasta according to package instructions, and drain.
Make the bechamel sauce. Melt the butter in a large pan, and add the flour. Mix it together with a spatula so that a paste forms. Then gradually add the milk and whisk until you get a creamy sauce.
Whisk in the seasonings; Italian seasoning, nutmeg, pepper, salt, and garlic. Add half of the cheese, and mix it in so that it melts.
Add the peas, sweetcorn, flaked tuna, and pasta. And mix everything well.
Pour the mixture into a baking dish (I used 20x30cm - 9x12 inches cast iron baking dish), and top with the rest of the grated mozzarella cheese. Bake uncovered for 15 minutes, then broil the top until it starts to brown (3-5 minutes).