1. Whisk milk, egg whites, honey, vanilla and salt in a large bowl.
2. Trim crusts off 8 bread slices and set aside. Cut the crusts and the remaining bread into 1-inch pieces. Toss the cut bread with dried apples, raisins, cinnamon and nutmeg in a large bowl.
3. Coat a 9-by-13-inch baking dish with cooking spray. Transfer the bread mixture to the pan. Lay the reserved crustless slices evenly over the top of the casserole, trimming to fit. Whisk the milk mixture one more time, then pour evenly over the bread. Press the bread down with the back of a wooden spoon, making sure it’s evenly moist. Cover with parchment paper, then foil, and refrigerate for at least 8 hours or up to 24 hours.
4. Preheat oven to 350°F.
5. Bake the casserole, covered, for 40 minutes. Uncover and continue baking until puffed, set and lightly browned, about 20 minutes more. Let stand for 10 minutes; dust with confectioner’s sugar and serve.