- 3 slices firm white sandwich bread, torn into quarters
- 1 1/2 sticks (3/4 cup) cold unsalted butter
- 2 cups all-purpose flour
- 1/2 cup packed light brown sugar
- 1/2 teaspoon salt
- 1 1/4 sticks (1/2 cup plus 2 tablespoons) unsalted butter
- 2 lb tart apples
- 2/3 cup granulated sugar
- 2 tablespoons all-purpose flour
- Preheat oven to 350°F.
- Grind bread to fine crumbs in a food processor. Spread in a shallow baking pan and toast in middle of oven, stirring once, until golden, about 5 minutes. Leave oven on.
- Cut butter into 1/2-inch pieces, then pulse in a food processor with flour, brown sugar, and salt until it begins to form small lumps. Sprinkle base into a 13- by 9-inch baking pan and press evenly onto bottom. Bake in middle of oven until golden, about 20 minutes.
- Melt butter and keep warm. Peel, quarter, and core apples and thinly slice quarters crosswise. Stir together sugar and flour in a small bowl.
- Sprinkle half of sugar mixture over hot shortbread, then top with apples and sprinkle with remaining sugar mixture. Top with bread crumbs and drizzle all of butter over them.
- Bake in upper third of oven, pressing down on filling with a metal spatula halfway through baking, until apples are very tender and bread crumbs are golden, 50 minutes to 1 hour total. Cool 20 minutes in pan on a rack.