In a large skillet, cook and crumble sausage over medium heat until no longer pink, 6-8 minutes. Remove with a slotted spoon; drain on paper towels. Discard drippings, reserving 1 tablespoon. Add apples to drippings; cook and stir over medium heat until tender, 4-6 minutes., In a bowl, combine sausage, apples, bread and cheese. Transfer to a greased 13x9-in. baking dish. In another bowl, whisk eggs, milk and mustard; pour over bread mixture. Refrigerate, covered, several hours or overnight., Preheat oven to 350°. Remove casserole from refrigerator while oven heats. Bake, covered, until a knife inserted in the center comes out clean, about 45 minutes. Uncover; bake until golden brown, 10-15 minutes longer. Let stand 5-10 minutes before serving.