Ask your butcher to bone the leg of lamb, saving the handle bone but having removed its stump. Open up the kidneys. Sear them in butter, over a medium heat. Sprinkle 1 teaspoon of mixed herbs, season with salt and pepper. Do not overcook the kidneys, they should remain rare. Stuff the leg of lamb with the kidneys, put the handle bone back in its place and tie up the leg into its original shape. Sliver the garlic and tuck into the meat. Weigh the leg of lamb. Rub the meat with the butter used for cooking the kidneys, season with salt and pepper and add a pinch of mixed herbs. Roast in a hot oven (440 F). Turn the leg of lamb to brown on all sides, then lower the heat to 410 F. Roast for 18 to 20 minutes per lb. Turn the oven off, keep the door ajar and let the meat rest for 5 minutes. Deglaze the roasting pan with a tablespoon of hot water, scrape all the meat drippings with a fork, bring to a boil on the stove. Whisk in the remaining butter, off the stove. Serve in a warmed sauceboat. Slice the leg of lamb just like an ordinary roast. Retrieve the juice and add to the sauce.
