01 - Preheat oven to 400°F. Place a rack in the lower third of the oven.
02 - Pat the lamb dry. Using a sharp knife, make small incisions all over the lamb. Insert garlic slices into the slits.
03 - In a small bowl, mix rosemary, thyme, lemon zest, salt, pepper, and olive oil. Rub this mixture thoroughly over the lamb.
04 - Arrange carrots, onions, and celery in a large roasting pan. Place the lamb on top, fat side up.
05 - Roast for 20 minutes at 400°F.
06 - Reduce oven temperature to 350°F, and continue roasting for 1 hour 25 minutes (for medium-rare), or until an internal temperature of 135°F is reached.
07 - Transfer the lamb to a platter, tent loosely with foil, and let rest for 15–20 minutes before carving.
08 - Serve slices of lamb with mint jelly on the side.