01 - Preheat oven to 400°F.
02 - Pat lamb dry. Using a sharp knife, make small incisions all over the lamb and insert slices of garlic.
03 - Rub lamb with olive oil, rosemary, thyme, salt, and pepper. Place in a large roasting pan.
04 - In a large bowl, toss carrots, parsnips, turnips, potatoes, and onions with olive oil, salt, and pepper. Scatter vegetables around the lamb in the roasting pan.
05 - Pour broth into the bottom of the pan, avoiding pouring directly over the lamb.
06 - Roast for 1 hour, basting occasionally. After 1 hour, check vegetables and stir for even roasting.
07 - Continue roasting for another 30 minutes, or until lamb reaches an internal temperature of 140°F for medium-rare.
08 - Remove lamb from oven. Transfer to a cutting board, tent loosely with foil, and rest for 15 minutes.
09 - Increase oven temperature to 430°F and return vegetables to oven for 10 minutes for extra caramelization, if desired.
10 - Carve lamb and serve with roasted vegetables and pan juices.