Cook onion in butter until tender, not brown. Stir in flour and pepper. Cook and stir until slightly thick and bubbly. Add cheese, stir until melted. Stir in cooked macaroni. Transfer to dish.
Melt 3 TBSP butter. Add 1 cup bread crumbs. Combine and top mac and cheese.
Bake at 350 degrees for 25-30 minutes.