Low Fat Tuna Casserole

1 box macaroni and cheese (use one that has powdered cheese)

2 tsp butter

1/4 cup chopped onion

1/4 cup skim milk

1 can cream of mushroom soup

2 cans tuna in water, drained

1 small can early young peas

Boil noodles according to package directions.  In a skillet, sauté onion in butter.  Mix in remaining ingredients, including noodles and pour all into a casserole dish.  Bake in preheated 350 oven for 20 minutes.