1. Heat oven to 350 degrees . Coat a 13 x 9 x 2-inch broiler-safe baking dish with nonstick cooking spray. Bring a large pot of lightly salted water to boiling. Add rigatoni and cook for 13 minutes, or as per package directions. Drain and return to pot.
2. Coat a large nonstick skillet with nonstick cooking spray and place over medium-high heat. Add sausage and cook 4 minutes, stirring until browned. Remove to a plate with a slotted spoon and keep warm.
3. Stir 1 tablespoon of the butter into same skillet. Add apples and onion and cook 5 minutes over medium heat. Remove from heat and stir in sausage, salt and pepper. Add to pasta in pot.
4. Meanwhile, melt remaining 2 tablespoons butter over medium heat in a medium saucepan. Whisk in flour and cook 1 minute. Pour in milk and mustard and bring to a simmer. Cook 3 minutes, whisking occasionally. Remove from heat and whisk in cheese. Stir into pasta mixture in pot.
5. Pour pasta mixture into prepared dish. Top with panko. Bake at 350 degrees for 25 minutes. Increase oven temperature to broil and broil dish for 3 minutes until lightly browned.