How to Make It
In a blender, puree 10 of the garlic cloves with the red wine vinegar, thyme and 1/2 cup of the water. Put the lamb in a roasting pan and pour the marinade over it. Cover and refrigerate overnight.
Preheat the oven to 325°. Spread the onion and the remaining 10 garlic cloves under the leg of lamb. Add the wine to the pan and rub the lamb with the olive oil. Top with the bay leaves and season with salt and pepper. Roast the lamb for about 4 1/2 hours, or until an instant-read thermometer inserted in the thickest part of the meat registers 160°. If the pan begins to look dry during cooking, add 1/2 cup of water.
Transfer the lamb to a carving board. In a small saucepan, combine the remaining 1/2 cup of water with the white wine vinegar, mint, butter and a pinch of salt and bring to a boil. Simmer over moderate heat until the broth resembles mint tea, about 3 minutes. Carve the lamb and serve with the mint broth.
Variation Make a pan sauce: Once the lamb is cooked, strain the contents of the roasting pan into a bowl, pressing hard on the solids. Skim off the fat. Set the roasting pan over 2 burners. Add 2 cups of water and boil over moderately high heat, stirring and scraping up the browned bits, until the liquid is reduced to 1 cup, about 5 minutes. Stir in the pan juices. Strain the pan sauce and season with salt.