To make the barley gruel soup, place the rolled barley in a pot with the cold soup or water and bring to a boil. Cover and simmer until the barley is really soft. Stir repeatedly during this process.
When the soup has the desired consistency, stir in butter and season with salt and a pinch of nutmeg, if desired. Mix egg yolk with cream or crème fraîche and thicken the no longer boiling soup with it. Serve the barley gruel soup sprinkled with chopped parsley.