Prep Time: 15 minutes
Cook Time: 30 minutes
Melt butter in medium saucepan over medium heat. Cook onion, if desired, in butter 5 minutes or until tender. Stir in flour, salt and pepper. Gradually add milk and cook, stirring occasionally, until thickened.
Remove from heat. Add 1-1/2 cups cheese and stir until cheese is melted. Combine cheese sauce with cooked macaroni. Pour into 1-1/2-quart casserole; top with remaining cheese. Bake in preheated 350°F oven 30 minutes or until bubbly and cheese is lightly browned.