Melt the butter in a saucepan, add the onion and fry until it is softened but not browned. Add the mushrooms and garlic and continue frying. The mushrooms will give off some liquid, so keep frying for a further 10 minutes or so, until the liquid is absorbed back into the mixture.
Add the sherry to the pan, increase the heat and bring to the boil to burn off the alcohol. Allow to cool slightly, then transfer to a blender. Blend to form a smooth paste, then add the stock and blend until smooth.
Add the parsley and cream and blend together. Return to the pan and heat gently. Serve with crusty bread.