01 - Preheat oven to 400°F. Position rack in the lower third of the oven.
02 - Pat the lamb completely dry with paper towels. Using a sharp knife, make small incisions all over the surface of the meat. Insert garlic slices into each slit.
03 - Combine rosemary, thyme, lemon zest, salt, pepper, and olive oil in a small bowl. Rub this mixture thoroughly over the entire surface of the lamb, pressing into the meat.
04 - Place carrots, onions, and celery in a large roasting pan to create a bed. Position the lamb on top of the vegetables, fat side facing up.
05 - Roast at 400°F for 20 minutes to develop a golden-brown crust.
06 - Reduce oven temperature to 350°F. Continue roasting for 1 hour 25 minutes for medium-rare, or until internal temperature reaches 135°F when measured with a meat thermometer inserted into the thickest part.
07 - Transfer the lamb to a carving platter. Tent loosely with aluminum foil and allow to rest for 15-20 minutes before carving. This redistribution of juices ensures tender, moist slices.
08 - Carve the lamb against the grain into thin slices. Arrange on plates and serve with mint jelly on the side.