01 - Preheat the oven to 425°F.
02 - Pat the lamb dry. Using a sharp knife, make small incisions all over the lamb and insert garlic slices.
03 - In a small bowl, mix rosemary, thyme, salt, pepper, and olive oil. Rub the mixture over the lamb.
04 - Place the lamb on a large roasting pan fitted with a rack.
05 - In a large bowl, toss carrots, parsnips, turnips, sweet potato, and red onion with olive oil, salt, and pepper. Spread vegetables evenly around the lamb in the roasting pan.
06 - Roast for 20 minutes, then reduce oven temperature to 350°F. Pour wine or broth around the vegetables if using.
07 - Continue roasting for 1 hour 15 minutes to 1 hour 25 minutes, or until the lamb reaches your desired doneness (135°F for medium-rare). Stir vegetables once or twice to ensure even roasting.
08 - Transfer the lamb to a cutting board, tent loosely with foil, and rest for 15 minutes before carving.
09 - Toss vegetables in the pan juices and serve alongside carved lamb.