01 - Preheat oven to 400°F. Pat the lamb dry, make small incisions all over, and insert garlic slices into each cut.
02 - Rub lamb with 2 tablespoons olive oil, then evenly coat with rosemary, thyme, salt, and black pepper.
03 - Place lamb on a rack inside a large roasting pan. In a bowl, toss carrots, parsnips, potatoes, sweet potato, onions with olive oil, salt, and pepper. Arrange vegetables around lamb in pan.
04 - Roast for 20 minutes at 400°F, then reduce oven temperature to 350°F.
05 - Roast at 350°F for 70 to 80 minutes until lamb reaches 140°F internal temperature for medium-rare. Stir vegetables halfway through for even browning.
06 - Transfer lamb to a cutting board, loosely tent with foil, and rest for 15 minutes.
07 - Return vegetables to oven if necessary to complete browning while lamb rests.
08 - Carve the lamb and serve alongside roasted vegetables, garnishing with parsley and accompanied by pan juices or gravy as desired.