It's worth the extra money to buy boneless half-leg of lamb (or ask the butcher to do it for you), as the whole leg has a complicated bone structure, and your knife skills must be pretty good to get the job done.
Be sure to trim as much fat as possible from the surface and any large nodules in the meat itself--lamb fat isn't very tasty. Banked grilling works best for leg of lamb, so the outside doesn't get burned before the inside is cooked. Use a meat-thermometer to judge the doneness of the thickest part of the lamb, knowing that the thinner parts will be more done.
2. In a blender or a food processor fitted with the metal blade, purée the onion, garlic, mint, oregano, toasted cumin, salt, and red pepper flakes. Transfer to a bowl and stir in the yogurt.
3. Using a thin bladed knife, trim off all excess fat from the surface of the lamb. With the tip of the knife, cut out any nodules of fat in the meat. Place the lamb on the work counter, with the outside of the lamb facing down. Make a few deep cuts in the thickest part of the lamb, and open them like a book to increase the surface the lamb.
4. Place the lamb and marinade in a zippered plastic bag and close. Refrigerate, turning occasionally, for at least 2 hours, and up to 8 hours. During the last hour, remove from the refrigerator.
5. Build a charcoal fire in an outdoor grill and let burn until covered with white ash. Protecting your hands with oven mitts, using a garden spade or another fireproof tool, spread the coals in a bank, with one side about two coals high, sloping to a depth of single coals. In a gas grill, preheat on High, then turn one burner to High, and the other burner(s) to Low.
6. Place the lamb over the hot area of the coals. Cover and grill until the underside is browned, about 5 minutes. Turn, cover, and grill until the other side is browned, 5 minutes more. Move the lamb to the low area of the coals and cover. For a gas grill, brown over the High burner, then transfer to the Low burner(s). Continue grilling until a meat thermometer inserted in the thickest part of the lamb registers 125°F for medium-rare meat (the thinner parts of the meat will be cooked to medium), 10 to 15 minutes.
7. Transfer to a carving board and let stand for 5 minutes. Slice across the grain into thin slices. Serve immediately.