Preheat oven to 400 degrees.
Bring a medium saucepan of water to a boil. Add macaroni and cook to al dente. Drain and set aside.
While the macaroni is cooking, melt butter in a medium saucepan over medium heat. Sauté onion in butter until soft. Stir in flour to form a paste and cook for 1 minute.
Reduce heat to medium-low. Whisk milk into flour mixture and slowly bring to a boil, stirring frequently. Reduce heat to low and stir in cheddar and American cheeses. Cook and stir over low heat until the cheeses have melted and the sauce is smooth.
Stir in dry mustard, Tabasco and salt and pepper.
Combine macaroni to cheese sauce.
Spray 6 mini casseroles/souffle dishes with cooking spray and place on a baking sheet. Divide macaroni and cheese evenly between dishes.
Sprinkle Parmesan cheese over the top of each dish.
Bake in preheated oven until hot and bubbly (about 20 minutes).