Preheat oven to 375 degrees. In a food processor, pulse bread until coarse crumbs form (you should have about 2 cups). Spread breadcrumbs on a rimmed baking sheet; bake until golden, 8 to 10 minutes. Let cool completely. Transfer to a bowl, add butter, and toss until coated.
Meanwhile, place apples in a large bowl, and toss with lemon juice. Stir in sugar, cinnamon, nutmeg, and half the breadcrumbs. Transfer mixture to an 8-inch square (or other shallow 2-quart) baking dish. Sprinkle with remaining breadcrumbs. Cover dish tightly with aluminum foil.
Bake until fruit mixture is bubbling, about 40 minutes. Then remove foil, and continue baking until breadcrumbs have browned and apples are easily pierced with a paring knife, 10 to 15 minutes more. Let cool at least 15 minutes before serving. Top, if desired, with vanilla ice cream sprinkled with cinnamon.