Preheat the oven to 400°F (200°C).
In a small bowl, combine garlic, rosemary, thyme, mustard, olive oil, salt, pepper, and lemon zest if using. Mix into a paste.
Pat the lamb dry with paper towels. If there’s excess fat, trim it, leaving a thin layer.
Make small incisions over the lamb with a sharp knife and rub the herb mixture all over, pressing it into the meat.
Place the lamb on a roasting rack in a pan. Roast at 400°F (200°C) for 20 minutes, then reduce heat to 350°F (175°C) and continue roasting for about 1 hour or until a meat thermometer inserted into the thickest part reads:
