Cook macaroni according to package directions. Drain. In a medium pot, melt butter and cook onion until soft. Add milk, cheese and mustard. Heat until bubbly and thickened slightly. Salt and pepper to taste. Add macaroni to cheese sauce. Pour into greased oven proof pan. Layer top with sliced tomato, bread crumbs and parmesan cheese. Bake 350' for 20-25 mins. until golden brown. Enjoy!