Preheat the oven to 400 degrees F, then combine mustard, garlic, rosemary, and pepper and rub the mustard mixture over lamb.
Add the roast to a shallow, foil-lined roasting pan and place the pan on meat rack and roast for 15 minutes.
Reduce the temperature of the oven to 325 degrees F and roast the lamb for about 20 minutes per pound for medium or until the internal temperature or the lamb reaches 145 degrees F when a meat thermometer is inserted into thickest part of roast and the temperature is tested.
Take roast from pan and transfer to cutting board, then cover roast with foil and let stand for 10 to 15 minutes before carving because the internal temperature of the roast will continue to rise 5 to 10 degrees F while it is standing.
Cut and discard the string from the roast, then carve into 20 slices and serve with mint jelly if desired.