1/4 c. chopped onion
2 tbsp. butter
1 can cream of mushroom soup
2 c. cheese
3 c. macaroni, cooked
1/2 c. milk
1/2 c. chopped canned tomatoes

In saucepan, cook onion in butter. Stir in soup, milk, and cheese. Heat until cheese melts - stir often. Add tomatoes and macaroni. Pour into 1 1/2 quart shallow baking dish. Bake at 350 degrees for 30 minutes.