Ask your butcher to bone the leg of lamb, saving the handle bone, without its stump. Open up the kidneys. Put them in a frying pan and sweat them in butter,on a low heat. Season with salt and pepper. Combine the thyme, rosemary and pesto and roll the kidneys into this mixture. Stuff the kidneys into the leg of lamb and place the handle bone back in its place. Sew and tie up the meat back to its original shape. Preheat the oven on maximum heat for 20 minutes. Place the leg of lamb on the roasting pan rack.
Brown on all sides for 15 minutes. Remove from the oven, leave to cool while preparing the pastry. Roll out a base of puff pastry (1/3 inch thick). Place the lamb on it. Roll out the rest of the pastry (1/4 inch thick) and wrap the leg of lamb, leaving the handle bone uncovered. It will serve as a chimney. Wet the edges of the pastry, seal them in a coil. Decorate the top and glaze with an egg. Bake on 350-410 F for 30 minutes without worrying about overcooking the lamb. Check for readiness by inserting a skewer right to the heart of the meat. The tip should come out hot.
