01 - Preheat the oven to 400°F.
02 - Pat the lamb dry and place it on a rack in a large roasting pan.
03 - In a small bowl, mix 3 tbsp olive oil, garlic, rosemary, thyme, salt, and pepper. Rub this mixture all over the lamb.
04 - Arrange the carrots, parsnips, turnips, onion, and potatoes in the bottom of the roasting pan. Drizzle with 2 tbsp olive oil, sprinkle with 1 tsp salt and ½ tsp pepper, and toss to coat evenly.
05 - Pour the broth around the vegetables.
06 - Roast the lamb and vegetables in the preheated oven for 1 hour.
07 - Reduce oven temperature to 350°F and continue roasting for 30 minutes, or until the lamb reaches desired doneness (internal temperature: 140°F for medium-rare).
08 - Transfer the lamb to a platter, cover loosely with foil, and let rest for 15 minutes before carving.
09 - Toss the vegetables in the pan juices and return to the oven if they need a little more browning for 10 minutes.
10 - Carve the lamb, serve with roasted vegetables, and garnish with lemon zest and chopped parsley.