1 (8 ounce) package processed American cheese, cut into strips
2 cups shredded sharp Cheddar cheese
1 (8 ounce) package elbow macaroni
1/4 tsp. ground black pepper
1/2 tsp. dry mustard
3/4 tsp. salt
2 cups milk
2 tbsp. all-purpose flour
1/4 cup finely chopped onion
2 tbsp. butter