Ask your butcher to bone the leg of lamb without cutting it open. Cream the butter with the finely chopped parsley and garlic, season with salt and pepper. Roll into a cylinder on a sheet of aluminum foil and refrigerate until firm. Push the cylinder (having peeled off the foil) into the meat and sew up the openings. Tie up the leg of lamb to keep its shape. Place it in a roasting pan in a hot oven (410 F) to sear the meat. Turn it 2 or 3 times towards the end of cooking. Turn the oven off, door ajar and let it rest for 5 minutes. Slice the lamb just like an ordinary roast. Deglaze the roasting pan with 1 or 2 tablespoons of boiling water. Serve this juice in a warmed sauceboat.
