Prepare macaroni as directed on package. Drain.In a medium saucepan melt butter and saute onion until tender.Add soup and milk. Stir until smooth.Add cheese and tomatoes. Cook stirring constantly until cheese melts. Combine macaroni into cheese mixture. Mix well
Pour into a 1 and a half quart casserole dish.Add a line of cheddar cheese on top. Bake at 350 degrees for 30 minutes. Serves 4 to 6