In a saucepan, melt 1 tablespoon butter; stir in flour, salt and pepper. Whisk in milk until smooth. Cook and stir for 2 minutes. Reduce heat to low. Add the cheese, mustard, Worcestershire sauce and onion; stir until cheese is melted. Add macaroni. Transfer to a greased 1-cup baking dish. Sprinkle with saltines; dot with remaining butter. Bake, uncovered, at 350° for 15 minutes or until heated through.