Soak the mushrooms in hot water for 15 minutes, drain and remove the stalks. Crush the garlic with the blade of a knife and put it in a hot oiled pan, cook for 2 or 3 seconds and remove from the pan. Put the mushrooms in the same pan and cook for 10 minutes. Now put the mushrooms in a saucepan with a quarter more than sufficient stock to cover, and the piece of ginger, crushed with a knife. Bring to the boil and simmer for 6 hours. Before serving, remove the ginger and season with a little salt. Serve with Chinese sauce.