Bone the leg of lamb and tie tightly with trussing string, so that it holds together and keeps its shape while cooking.
With a sharp, long-bladed knife make a deep incision into the leg and insert a mixture of chopped sage, rosemary and garlic.
Wash, peel, seed the tomatoes and pass through a food mill. Heat the oil in a large skillet with high sides over medium heat, add the lamb and season with salt and pepper. Turn the heat up to brown the meat quickly and evenly on all sides, then add the finely chopped onion and cook for about 10 minutes, stirring every once in a while.
Stir the tomatoes into the lamb and cover. Cook over moderate heat for 45 minutes. Check often, adding spoonfuls of warm water, if necessary, to prevent the lamb from sticking to the pan. Season with salt and pepper and remove from the heat. To serve, slice the lamb and arrange on a hot platter. Top it with the hot sauce and serve immediately.