QUICK TUNA CASSEROLE

1 (14 oz) pkg macaroni and cheese dinner
1 small onion, chopped
1 can cream of mushroom soup
1/2 cup milk
1 can (6 1/2 oz) tuna
butter
1/2 cup crushed potato chips
1/2 cup grated Cheddar cheese
Prepare macaroni and cheese dinner as directed on package. Add onion, soup, milk and tuna; mix. Butter casserole dish; add tuna mixture. Top with chips and cheese.
Bake at 350° for 20 minutes.