Combine the yogurt, lemon juice, olive oil, scallions, minced garlic, mint, thyme, lemon zest, pepper, and salt in a large bowl and stir to mix.
Rub the garlic clove over both sides of the lamb and put the lamb in a shallow glass or ceramic dish. Pour the marinade over the meat and turn it several times to coat. Cover and refrigerate for 4 to 6 hours. About 30 minutes before grilling, remove the meat from the refrigerator and let it come to room temperature.
Prepare a charcoal or gas grill arranging the coals for indirect cooking. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be hot.
Lift the lamb from the marinade, scrape off the excess marinade, and season both sides with salt and pepper. Discard the marinade.
Combine the oil and crushed garlic in a small bowl for basting the meat.
Sear the lamb over the hot coals for about 5 minutes on each side, basting with the olive oil and garlic mixture frequently during the first 20 minutes of grilling. Move the lamb to the cooler part of the grill, cover, and cook for about 20 minutes. Turn the meat over, cover and cook for 20 to 25 minutes longer for medium-rare meat, or until cooked to the desired doneness. An instant-read thermometer inserted in the thickest part of the meat should register 140°F for rare meat and 145° to 150°F for medium-rare. Let the lamb rest for about 10 minutes before slicing.