Pat lamb dry and score the fat in a diamond pattern.
Rub lamb with olive oil, mustard, minced garlic, rosemary, thyme, salt, and pepper.
Place lamb in a large foil tray and arrange potatoes, carrots, onions, and halved garlic around it.
Preheat grill to 300°F for indirect heat.
Set tray on the cooler side of the grill, close the lid, and cook 2 hours.
Baste with pan juices and continue cooking 1.5–2 more hours until bone pulls free and internal temperature reaches 195°F.
Remove from grill, tent with foil, and rest 20 minutes.
Shred the meat and serve with the roasted vegetables.
Tip: Keep the grill lid closed as much as possible to maintain steady temperature.