Preheat oven to 350 degrees.
Cook macaroni in boiling, salted water - according to package directions - until tender. Drain.
In another saucepan, melt butter. Add onion; cook until soft.
Blend in flour and milk.
Cook and stir until thick.
Add salt, pepper and cheese, heating and stirring until cheese is melted.
Add the cooked and drained macaroni to the cheese sauce. Stir to combine.
Pour the macaroni/cheese mixture into greased 1 ½-quart casserole.
Sprinkle with additional cheese and paprika.
Top, if desired, with sliced tomatoes, sprinkled with salt.
Bake 30 minutes at 350 degrees or until bubbly and browned.