Step 1To broil, cook the meat until browned on both sides and finish in a 350-degree oven if necessary. Remember that the goal is still 140 degrees in the thickest part of the meat.
Step 2To grill, use the indirect heat method, banking the coals or lighting the burners on only one side of the grill. Cook the meat, with a drip pan underneath, on the opposite side of the grill. The temperature inside the covered grill should be around 425 degrees. Because the meat is only about 2 inches thick, it will cook quickly; a 5-to 6-pound butterflied leg will take 45 to 55 minutes. Just before the lamb is done, the meat can be moved directly over the heat to brown if necessary. Be careful of residual fat, which will cause the fire to flame up.
Step 3Carefully trim away the tough outer layer of fat (fell) from the lamb. Rub the lamb with garlic, thyme and salt and pepper to taste. Place the lamb in a large plastic food bag. Pour the wine over and seal the bag tightly. Marinate the lamb in the refrigerator overnight, turning occasionally to coat the lamb evenly with the wine.
Step 4Roast the reserved hip bone and any lean trimmings in a greased roasting pan at 450 degrees until well browned. Cook the turnips in rapidly boiling water just until a knife easily pierces the center, about 5 minutes. Remove them from the water and refresh in ice water.
Step 5Place the hip bone and trimmings, 1 quartered onion and 2 chopped carrots in a medium saucepan. Cover them with water and bring slowly to a simmer. Cook, keeping at a bare simmer, at least 1 hour. When done, strain and chill.
Step 6The next day, remove the lamb from the marinade, reserving the marinade, and pat the meat dry with paper towels. Place the lamb on a rack in a greased roasting pan and scatter the remaining onion and carrots around the bottom of the pan. Roast the lamb at 325 degrees until the internal temperature reaches 135 degrees. Allow 20 to 25 minutes per pound. When the lamb is done, remove it from the roasting pan and place it on a serving platter or carving board loosely tented with foil to keep it warm, at least 1/2 hour.
Step 7While the lamb is roasting, remove the stock from the refrigerator, skim the fat and return it to a simmer along with handful of reserved pea pods and any reserved turnip peels. Cook at least 1/2 hour.
Step 8In a large saute pan, combine the peas, cooked turnips, 1/4 cup butter, shallot, lettuce, thyme leaves and 1/4 cup lamb stock. Place over medium heat and cook just until the peas are no longer starchy, about 5 minutes.
Step 9While the peas are cooking, remove the onions and carrots and skim the fat from the meat juices left in the roasting pan. Place the pan over high heat and add the reserved red wine marinade. Cook, scraping the bottom, until the marinade reduces to several tablespoons, about 5 to 10 minutes. Add 1 cup lamb stock and cook until the mixture is reduced to a thin sauce, about 10 minutes. Keep warm.
Step 10When ready to serve, carve the lamb (pouring the juice into the warm sauce) and place on a platter. Bring the sauce to a boil and whisk in 2 tablespoons of butter. Strain the sauce into a sauceboat and serve the peas and turnips alongside the lamb.