01 - Preheat your oven to 400°F, ensuring it reaches the proper temperature before beginning the roasting process.
02 - Pat the lamb completely dry with paper towels. Using a sharp knife, create small incisions all over the surface of the meat and insert sliced garlic cloves into each cut.
03 - In a mixing bowl, combine the chopped rosemary, fresh thyme leaves, kosher salt, black pepper, olive oil, lemon zest, and lemon juice. Stir until well incorporated.
04 - Rub the herb mixture thoroughly over all surfaces of the lamb, pressing gently to ensure the marinade adheres well to the meat.
05 - Place the seasoned leg of lamb in a large roasting pan, fat side up for optimal rendering and flavor development.
06 - In a separate bowl, toss the carrots, parsnips, turnips, red onions, and baby potatoes with 2 tablespoons olive oil, salt, and pepper until evenly coated.
07 - Distribute the seasoned root vegetables around the lamb in the roasting pan, ensuring they are in a single layer for proper caramelization.
08 - If using, pour the white wine or chicken broth into the bottom of the roasting pan to create steam and enhance pan juices.
09 - Roast the lamb and vegetables at 400°F for 1 hour. Stir the vegetables halfway through cooking to promote even browning.
10 - Reduce the oven temperature to 350°F and continue roasting for an additional 30 to 40 minutes, or until the lamb reaches your desired doneness (140°F internal temperature for medium-rare).
11 - Remove the roasting pan from the oven. Tent the lamb loosely with aluminum foil and let it rest for 15 minutes to allow juices to redistribute throughout the meat.
12 - Carve the rested lamb against the grain into thin slices. Arrange on serving plates alongside the caramelized root vegetables and drizzle with pan juices if desired.