Allow 1 pound per serving and add a little extra cooking time if the roast is boneless.
Preheat the oven to 500 F. Make slits in the meat and insert the slivers of garlic. Rub the surface of the lamb with the mustard and season generously with pepper. Place the lamb into a roasting pan and cook for 20 minutes. Reduce the oven temperature to 350 F and remove the pan from the oven. Add the potatoes and rutabaga around the lamb, and coat the vegetables with the pan drippings. Return the pan to the oven and roast for 50 to 70 minutes more (depending on the bone), or until a meat thermometer reads 145 F (medium rare), basting occasionally with the pan juices.