Prepare corn muffin mix according to package directions. Cool and crumble. Preheat oven to 350 ºF. Place sausage in large, deep skillet. Cook over medium high heat until evenly brown. Drain and set aside. In another large skillet over medium heat, slowly cook and stir the mushrooms, celery, onion and garlic until soft. In a large bowl, mix the crumbled prepared corn muffin mix, sausages, mushroom mix, apple, bread, salt and pepper. Transfer the mixture to a medium baking dish. Bake in the preheated oven 45 minutes, or until lightly browned.