Cook macaroni for 10 minutes. Drain well. In med. saucepan, cook onion in margarine or butter until tender, but not brown. Stir in flour and pepper.
Add milk all at once. Cook and stir until slightly thick and bubbly. Add cheese, stirring until melted. Stir in macaroni. Put in 1 qt. casserole. Bake in 350 degree oven for 25-30 min. If desired, arrange tomato slices atop macaroni for the last 5 minutes of baking. Let stand 10 min. before serving.