Cook pasta according to package directions. Drain. Keep warm.

Meanwhile, in medium saucepan stir together flour, salt, pepper and mustard. Whisk in 2 tablespoons of the milk, forming smooth paste. Gradually whisk in remaining milk. Add cream cheese. Cook and stir over medium heat until thickened and boiling. Remove from heat. Gradually stir in Cheddar cheese and American cheese, a little at a time, until melted. Fold in pasta and 2 tablespoons onion (if desired).

Spread in 8 x 8 x 2-inch baking dish sprayed with cooking spray. Sprinkle with Kellogg's® Rice Krispies® cereal and Parmesan cheese. Broil 4-to-5 inches from the heat about 3 minutes or until crisp and heated through. Serve topped with additional onions (if desired).

For a different flavor, try stirring in one of the following: 1 cup cooked and well-drained, chopped broccoli; 1 cup chopped, fully-cooked ham; 1 cup thinly sliced, fully-cooked, smoked turkey sausage; 1 cup cooked and well-drained peas; or 1 cup cooked and well-drained, chopped cauliflower into the cheese mixture along with the cooked pasta and onion.