Heat oven to 350°F. Spray 2-quart casserole with cooking spray. Cook and drain pasta as directed on package.
Meanwhile, in 12-inch skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until beef is brown; drain.
Stir pasta, 3 tablespoons of the parsley, the milk, barbecue sauce, mozzarella cheese and 1 cup of the Cheddar cheese into beef in skillet. Spoon mixture into casserole. Sprinkle with remaining 1/2 cup Cheddar cheese.
Bake uncovered 30 to 40 minutes or until hot and bubbly. Sprinkle with remaining 1 tablespoon parsley.