Fill a large pot with 4 quarts water and put on high heat to boil.

Chop onion and celery and saute in a little oil or spray. (Note: Feel free to add mushrooms or peppers, I can barely get away with celery with my family's tastes).

Preheat oven to 350 degrees.

Add tortellini to water and cook per package directions - you can undercook a little.

In a 2 quart casserole, mix tuna, soup, and milk.

Add celery and onion mixture to casserole.

Drain tortellini and add to casserole. Add cheese and fold in with tuna mixture.

Sprinkle with crushed potato chips, bread crumbs, etc., if you desire.

Bake for 20 minutes or until heated through.