Put oven rack in middle position and preheat oven to 350°F.
Cover prunes with boiling-hot water in a bowl and let stand 15 minutes. Drain and quarter prunes, then toss with apples and lemon juice. Stir together crumbs and butter in another bowl. Stir together brown sugar, cinnamon, and salt in a separate bowl. Spread one third of crumbs in a 9-inch pie plate. Cover with half of apple mixture, then sprinkle with half of sugar mixture. Sprinkle with half of remaining crumbs and cover with remaining apples, then remaining sugar mixture. Top with remaining crumbs.
Bake, covered with foil, until apples are tender, about 40 minutes. Increase oven temperature to 425°F, remove foil, and bake until top is golden brown, about 10 minutes more. Serve warm.
