Cook pasta according to package directions. Preheat oven to 350°. In a medium saucepan, melt margarine. Stir in flour. Gradually stir in milk. Cook, stirring constantly over medium heat, until sauce thickens. Stir in salt and pepper. Add onion and cheese; stir until cheese melts. Stir cheese sauce into pasta. Spoon into a greased 2-quart baking dish. Top with breadcrumbs. Bake 30 minutes. Tip: Recipe may be doubled.