Heat oven to 425 degrees F. Coat 13 x 9 x 2-inch baking pan with nonstick cooking spray.
Lightly beat together egg whites and whole eggs in bowl. Stir in milk, mustard, salt and pepper.
Toss together bread, sausage and sage in large bowl. Transfer to baking dish. Top with apples. Slowly pour egg mixture into baking dish. Sprinkle with cheese. Cover with foil. Let stand 10 minutes or refrigerate for up to 10 hours. If refrigerated, remove strata to counter for 30 minutes before baking.
Bake, covered, in 425 degree F oven for 10 minutes. Uncover; bake 15 minutes or until set and top is lightly browned. Let stand 10 minutes. Makes 8 servings.