Melt the butter in a pan, add the mushrooms and allow to soften. When the juices begin to run, add the garlic, 1 tablespoon of the parsley, salt and pepper and grated nutmeg, and continue cooking for several minutes.
Soak a thick slice of crustless break in a little stock while the mushrooms are cooking, then squeeze out the moisture and add to the mushrooms.
Stir until the bread amalgamates (don't worry about the lumps). Add the stock and cook for 15 minutes.
Blend the soup and return to the pan to reheat. Add boiling cream (if using) and sprinkle with the remaining tablespoon of parsley.