Place all the vegetables in a roasting pan. Place the garlic cloves in among the vegetables. Pour the beer, stock, and olive oil over the vegetables. Sprinkle with the rosemary and season with salt.
Pat the meat dry with paper towels. Rub it all over with 1 tablespoon salt. If not using honey mustard, combine the Dijon mustard and honey in a small bowl. Rub the leg with the honey mustard or mustard-and-honey mixture. Insert a meat thermometer, if you have one, into the thickest part of the leg; make sure it does not touch the bone.
Place the leg of lamb on a rack that will fit over the roasting pan. Place the roasting pan on the bottom rack of the oven and set the rack with the lamb over the pan, so the roasting juices will drip down onto the vegetables. Cook for 15 minutes, then reduce the heat to 350°F. Cook for another 1½ hours, or until the meat thermometer (or an instant-read thermometer) reads 145°F. Make sure the vegetables do not get dry; if they look dry, add another cup or so of beer or stock.
Let the meat rest, uncovered, on a carving board for at least 30 minutes. Leave the vegetables in the oven to keep warm.
When the meat is ready, transfer the vegetables to a serving platter. I like to carve the meat at the table, but you may choose to carve it in the kitchen; make sure the serving platter is warm (but not hot). The muscle fibers on a leg of lamb run more or less parallel with the bone. For a more tender texture, slice the meat across the muscle fibers. Serve the lamb with the root vegetables; be sure to squeeze the softened garlic out of the skins and spread it on the meat as you eat.