In a medium saucepan, bring the raw lamb, if using, and the stock to a boil. Lower the heat and simmer for 15 minutes. Stir in the potatoes. Return to a boil. Lower the heat, stir in the cooked lamb, if using, and simmer for 15 minutes.
Meanwhile, in a medium frying pan, melt the butter. Stir in the sorrel and cook over medium heat, stirring, until the sorrel has wilted and changed colo