Boil the pasta as instructed on the package. I made 3/4 of a package of rigatoni and mine went for about 13 minutes. When they are done, drain but don't rinse.
1 1/2 cup mushrooms
1 cup onion
4 tbsp butter
4 tbsp flour
3 1/2 cup milk
salt and pepper
1 cup shredded mozzarella
2 cans tuna
Saute the mushrooms and onion in the butter for 10 minutes. Sprinkle over some salt just to help them release the water. Once the onions are very translucent and the mushrooms have shrunk into tiny pieces, add in the flour. Cook for another 2 minutes then start adding in the milk.
Fold in the drained pasta and pour into a baking dish.
Sprinkle over the rest of the cheese and bake in a 400 degrees oven for 20 minutes. The cheese should be melted and some parts should have turned a golden brown.