How to Make It
In a skillet, toast the fennel seeds over moderate heat for 2 minutes; crush and transfer to a medium bowl. In a food processor, finely chop the fresh fennel and garlic.
In a skillet, heat 3 tablespoons of the oil. Add the fennel-garlic mixture and cook over moderately low heat until softened, 10 minutes. Add to the bowl with the minced rosemary, crushed red pepper, cracked peppercorns and 1 tablespoon of salt. Let cool, then mix in the ground lamb.
Preheat the oven to 425°. Spread the saddle of lamb on a work surface, fat side up. Using a knife, lightly score the fat crosswise at 1/2-inch intervals. Turn the saddle over and season with salt and pepper. Spread all but 2 cups of the lamb filling over the meat; reserve the rest for another use. Roll up the roast, wrapping the flaps of fat around the outside and forming a neat cylinder, then tie at 1-inch intervals with kitchen string. Season with salt and pepper.
In a large ovenproof skillet, heat the remaining 2 tablespoons of oil. Add the roast and cook over moderately high heat until browned, about 10 minutes. Add the rosemary sprigs to the skillet. Transfer the lamb to the oven and roast for 1 hour 15 minutes, or until an instant-read thermometer inserted in the center of the lamb (not the filling) registers 130°. Transfer to a carving board. Let rest for 15 minutes.
Meanwhile, pour off the fat in the skillet. Add the stock and simmer over moderately high heat for 3 minutes. Add the butter and simmer for 2 minutes. Strain the sauce into a bowl; whisk in the mustard. Season with salt and pepper.
Discard the strings from the roast. Slice the roast crosswise and serve with the mustard pan sauce.
Order the lamb saddle from the butcher.