Using a sharp knife, make small slits in the fat. Remove the meat from the bone (or have your butcher do it for you, reserving the bone). Season well with salt and pepper. Squeeze over the juice of the lemon to prevent fat flare-ups. Tie the bones to the meat with butcher's twine. Place 2 to 3 cabbage leaves on the meat side of the roast. Tie the leaves on the roast with butcher's twine in several places to secure the leaves and prevent the meat from burning. Prepare a charcoal or gas grill, arranging the coals for indirect cooking. Lightly spray the grill rack with vegetable oil spray. The coals should be moderately hot to hot. Sear the meat over intense heat on the bone side and the narrow sides for 1 or 2 minutes per side. Sear the meaty side (top) for about 3 minutes. When turning the meat, use tongs or other utensils to avoid piercing the meat more than necessary to keep as much moisture inside the roast as you can. Transfer the roast, bone-side down, to the cooler part of the grill. Cover the grill and cook for 25 minutes, then baste and turn the roast, cooking for another 15 minutes. Remove the cabbage leaves after about 40 to 45 minutes. Continue grilling, turning and basting occasionally with beer, for a total of 1 hour and 10 minutes to 1 hour and 20 minutes for rare. After 1 hour, begin checking the meat by inserting and instant-read thermometer into the thickest part of the meat, taking care not to insert the thermometer too close to a bone. When the thermometer registers 135 to 140 degrees F, remove the meat from the grill, and let rest for 5 to 10 minutes before carving.
Heat the olive oil in a medium skillet over medium-high heat until bubbling and hot, about 1 minute. Remove from the heat and add the wine, salt, pepper, garlic, butter, parsley, mushrooms and demi-glace. Cook, stirring often, until the mushrooms are tender and the sauce is thick and reduced enough to coat the back of a spoon, 5 to 7 minutes. Yield: about 1 1/2 cups