Heat oven to 375°F.
Melt butter in medium Dutch oven over medium heat.
Add breakfast sausage and sauté about 7 minutes. With slotted spoon, remove and set aside. Leave sausage drippings in pot.
Reduce heat to low and add flour to pan drippings. Stir until very thick, dry mixture forms.
Add milk 1/2 cup at a time, whisking constantly until smooth and bringing almost to boiling point, but do not boil. Stir in cheddar, mustard, salt and nutmeg. Keep warm.
Meanwhile, cook pasta according to package directions; drain and toss with olive oil.
Stir apple cubes, reserved sausage and pasta into cheese sauce.
Bake 30 to 40 minutes until brown and bubbly. Let sit 15 minutes before serving.