Put the stock on to boil. Soak the bread in a little cold water.
Peel and chop the onion or shallot. Clean and roughly chop the mushrooms. Chop the parsley.
Heat the oil in a large pan. Fry the onion or shallot over moderate heat until lightly browned. Peel the garlic and crush it into the pan. Add the mushrooms and cook until they release their liquid, then add the chopped parsley.
Squeeze as much water as possible from the bread, then stir it into the mushrooms. Add the stock and mace or nutmeg. Return to the boil, half cover the pan and simmer for 15–20 minutes.
Purée or blend the soup until it is creamy but still slightly grainy, then reheat, season with salt and pepper and serve immediately.