Season the roast well with salt and pepper. Squeeze the lemon juice over the fat to prevent flare-ups.
Place 2 to 3 cabbage leaves on the meat side of the roast. Tie the leaves onto the roast with twine in several places to prevent the meat from burning.
Prepare a charcoal or gas grill, arranging the coals for indirect cooking. Lightly coat the grilling rack with cooking spray. The coals should be moderately hot to hot.
Over the hottest part of the grill, sear the meat on the bone side and the narrow sides, 1 to 2 minutes per side. Sear the meat side, about 3 minutes. Use tongs when turning roast to avoid unnecessary punctures, so the meat retains as much moisture as possible.
Transfer the roast, bone side down, to the cooler part of the grill. Cover the grill, and cook 25 minutes. Baste with ale, and turn roast; cook 15 minutes more. Continue grilling, turning and basting occasionally with ale. Remove the cabbage leaves after 45 minutes. One hour into the cooking time, insert an instant-read thermometer into the thickest part of the meat, not too close to a bone. Continue cooking until thermometer registers 130° for rare or 140° for medium, 1 hour and 10 minutes to 1 hour and 20 minutes. Remove roast from grill, and let rest 5 to 10 minutes before carving.
Prepare Leon's Sauce.