Make sure your shells are prepared, place in a bowl with cold water.

Dice the onion, add to a larger pan, and lightly sauté them, add flour, then gradually add milk.

In a separate bowl combine bread crumbs, Parmesan, then divide the mixture in two.

Add drained tuna to one half the bread mixture. Add mozzarella cheese, And add half the milk mix.

Stir well, begin filling each noodle with the tuna mixture, placing in a baking dish.

Once it is full, tip it with the remaining milk mix, and then sprinkle some Parmesan grounds, and mozzarella, and the remaining bread crumbs.

Place in oven on 350 until sauce begins to bubble, about 10 minutes.