Cook macaroni according to package directions; drain.
In saucepan melt butter or margarine. If using onion, cook it in butter till tender but not brown. Blend in flour, the 1/2 teaspoon salt, and pepper. Add milk all at once; cook and stir till thickened and bubbly. Add cubed Kraft Cheese to sauce; stir till melted. Stir cheese sauce into macaroni.
Turn mixture into a 1 1/2 quart casserole. Sprinkle tomato slices with a little salt; arrange atop macaroni.
Bake, uncovered at 350 degrees F till heated through, 30 to 35 minutes.