01 - Set oven to 400°F for initial high-heat roasting.
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 incision.
03 - Combine rosemary, thyme, salt, pepper, olive oil, lemon zest, and lemon juice in a bowl. Mix thoroughly to form a paste.
04 - Rub the herb mixture evenly over the entire surface of the lamb, pressing gently to adhere.
05 - Place the seasoned lamb in a large roasting pan, fat-side up.
06 - Combine carrots, parsnips, turnips, onions, and potatoes in a bowl. Drizzle with olive oil and season with salt and pepper. Toss to coat evenly.
07 - Arrange the seasoned root vegetables around the lamb in the roasting pan.
08 - If using white wine or broth, pour it into the bottom of the roasting pan.
09 - Roast at 400°F for 1 hour. Stir the vegetables halfway through cooking to ensure even browning.
10 - Reduce oven temperature to 350°F. Continue roasting for 30 to 40 minutes, or until lamb reaches internal temperature of 140°F for medium-rare doneness.
11 - Remove lamb and vegetables from oven. Tent lamb loosely with aluminum foil and let rest for 15 minutes before carving.
12 - Carve lamb against the grain and serve alongside the roasted root vegetables.