Preheat the oven to 425 degrees F. Place the lamb, meaty side up, in a roasting pan and brush with mustard. Sprinkle with rosemary, salt and pepper. Roast 22 to 25 minutes for rare, 25 to 28 minutes for medium rare, and 30 minutes for medium.
When the lamb is almost done, place sauce ingredients, except cornstarch mixture, in a small pot and set over medium to medium-high heat. Bring to a simmer and whisk until the jelly is melted and well combined with the other ingredients. Stir in the cornstarch mixture, return to a simmer, and cook until sauce lightly thickens. Cover and reserve on low heat.
When cooked, set the lamb on a plate, tent with foil, and rest 10 minutes. Slice the lamb into single chop pieces and arrange on a platter with a bowl of the sauce for dipping.
Note: Red currant jelly is sold in the aisle jams are sold. If unavailable, you could try another preserve in this recipe, such as blackberry jam or cherry jam.