1. Adjust oven rack to middle position and heat oven to 325°F. Pulse half of bread, 1 tablespoon butter, 1/2 teaspoon salt, and 1/4 teaspoon pepper in food processor until finely ground, 10 to 15 pulses. Transfer to rimmed baking sheet. Repeat with remaining bread, 1 tablespoon butter, 1/2 teaspoon salt, and 1/4 teaspoon pepper and add to sheet. Bake crumbs until golden brown, 15 to 20 minutes, stirring halfway through baking. Transfer to large bowl; set aside.

2. Meanwhile, melt remaining 2 tablespoons butter in 12-inch skillet over medium heat. Add apples, onion, 1 teaspoon salt, and 1/2 teaspoon pepper and cook until apples are soft and onion is translucent, 7 to 10 minutes. Add prunes, sage, garlic, and fennel and cook until fragrant, about 1 minute. Stir apple mixture into bread crumbs until well combined.

3. Line rimmed baking sheet with aluminum foil. Combine sugar, 1 tablespoon salt, and 1 tablespoon pepper in bowl. Pat ribs dry with paper towels and rub all over with sugar mixture. Lay five 20-inch-long pieces of kitchen twine about 3 inches apart crosswise on prepared sheet. Lay 1 rib rack, meat side down, across twine. Place stuffing mixture on top of rack and pack to uniform thickness. Place remaining rib rack, meat side up, on top of stuffing, arranging wider end of rack over tapered end of bottom rack to sandwich stuffing. Tie racks together and trim excess twine.

4. Roast ribs until tender and well browned, about 3 hours, flipping bundle halfway through roasting. Transfer ribs to carving board and let rest for 15 minutes. Cut ribs between bones, creating individual stuffed rib portions; discard twine. (Alternatively, discard twine and remove top rack of ribs. Transfer stuffing to platter. Cut ribs in between bones to separate and transfer to platter with stuffing.) Serve.

Serves 6 to 8