01 - Preheat the oven to 400°F.
02 - Pat the lamb dry. With a sharp knife, make small incisions all over the leg and insert garlic slices.
03 - In a small bowl, mix rosemary, thyme, mustard, salt, pepper, and olive oil to form a paste.
04 - Rub the herb paste all over the lamb, massaging it into the incisions as well.
05 - Place the lamb on a rack in a roasting pan. Roast for 20 minutes, then reduce oven temperature to 340°F.
06 - Continue roasting for 1 hour 10 minutes, or until a meat thermometer inserted into the thickest part reads 140°F for medium-rare.
07 - Remove the lamb from the oven, cover loosely with foil, and let rest for 15-20 minutes before carving.
08 - Serve sliced lamb with mint jelly on the side. Garnish with fresh mint if desired.