How to Make It
In a medium skillet, cook the bacon over moderately high heat for 3 minutes. Add the onion and garlic and cook over moderate heat until softened, about 7 minutes. Transfer to a bowl and let cool. Add the walnuts, bread crumbs, egg, milk, rosemary, 3/4 teaspoon salt and 1/2 teaspoon pepper.
Trim any excess fat from the lamb, being sure to leave the loins and flaps attached and intact. Place the meat on a work surface, fat sides down, and season with salt and pepper. Pack half of the stuffing along the length of each loin, in a loose sausage shape. Roll up the flaps, covering the stuffing, and tie each loin securely with kitchen string.
Preheat the oven to 500°. Heat a large heavy nonreactive skillet. Add the roasts and cook over high heat, turning occasionally, until browned and crusty on all sides. Pour off any fat as it accumulates in the pan.
Transfer the lamb to a roasting pan and roast for 20 to 25 minutes, or until an instant-read thermometer registers 130° when inserted in the center of the meat (for medium rare). Transfer the lamb to a cutting board, cover loosely with foil and let rest for 10 minutes before carving.
Meanwhile, add the beef stock and the reserved compote juices to the skillet and bring to a boil, scraping up any browned bits clinging to the skillet. Cook until the sauce is slightly thickened and reduced to 3/4 cup, about 5 minutes.
Using a sharp knife, cut off the strings from the roasts. Cut each loin into 8 slices and arrange 2 slices on each of 8 warmed dinner plates. Spoon some of the sauce over the lamb, sprinkle with chopped rosemary and serve.