Preheat oven to 450 degree F. Unfold piecrust and use to line a 9-inch pie plate. Bake according to package directions. Set aside. Reduce oven temperature to 350 degree F.
In a large skillet cook apples, onion, and sugar (if using) in hot butter over medium heat for 5 minutes or until tender. Transfer mixture to a medium bowl. In the same skillet cook the sausage and garlic until meat is no longer pink; drain in colander. Pat sausage dry with clean paper towels. Spoon sausage into prebaked crust. Top with the cooked apple mixture. Combine cheeses and eggs; spoon evenly over apple mixture.
Bake for 35 to 40 minutes or until a knife inserted near the center comes out clean. Let stand 10 minutes on a wire rack before serving. Makes 8 servings.