01 - Preheat oven to 430°F.
02 - Use a sharp knife to make small slits all over the leg of lamb. Insert garlic slices into the slits.
03 - Combine rosemary, thyme, salt, pepper, and olive oil in a small bowl to form a paste. Rub evenly over the lamb.
04 - Place lamb on a rack in a roasting pan. Roast for 20 minutes at 430°F, then reduce heat to 350°F and continue roasting for 1 hour 10 minutes for medium-rare, or until internal temperature reaches 135°F.
05 - Remove from oven, cover loosely with foil, and let rest for 15–20 minutes before carving.
06 - While lamb rests, mix sugar and boiling water until dissolved. Stir in mint, vinegar, and salt. Let stand for 10 minutes to infuse flavors.
07 - Carve the lamb and serve with the mint sauce.