Heat oven to 350. spray a 13" x 9" broiler-safe baking dish w/ cooking spray. Cook pasta as directed, drain.
Spray a lg skillet w/ cooking spray, place over med-high heat. Add sausage, cook about 4-5 min. Remove to plate, keep warm. Stir 1 tbsp butter into skillet. Add apples and onion, cook 5 mins over medium heat. Remove from heat and stir in sausage, salt & pepper. Add to pasta in pot.
Melt remaining butter over medium heat in a lg saucepan. Whisk in flour, cook 1 min. Pour in milk and mustard. Bring to a simmer and cook 3 min, whisking occasionally. Remove from heat and whisk in cheese. Stir into pasta mix. Pour pasta mix into baking dish. Top w/ panko bread crumbs. Bake for 25 mins. Set oven temp to broil and broil until browned, about 3 mins.