Preheat the oven to 450°. Season the lamb with salt and pepper and put it in a medium roasting pan. Scatter the onions and carrots around the meat and roast for 15 minutes. Reduce the oven temperature to 325° and roast the lamb until an instant-read thermometer inserted in the thickest part of the meat registers 120°, about 1 1/2 hours. Leave the oven on. Using a pastry brush or spoon, spread Tart Cherry Apple Rosemary Glaze all over the meat, roast for 10 minutes longer, or until an instant-read thermometer inserted in the thickest part of the meat registers 130°. Transfer the lamb to a cutting board and let rest for 20 minutes. Transfer the vegetables to a bowl and keep warm.