While pasta is cooking, make cheese sauce. In 3-quart saucepan, melt butter over medium heat. Stir in flour, salt, pepper and mustard with wire whisk. Cook about 2 minutes, stirring constantly, until mixture is smooth and bubbly; remove from heat. Gradually stir in milk. Heat to boiling, stirring constantly; boil and stir 1 minute. Remove from heat; stir in caramelized onions and all remaining cheese sauce ingredients.