Sauté the mushroom with the butter in a heavy saucepan for a few minutes. Add the parsley and cook, stirring, until soft.
Add the breadcrumbs and garlic, stir and pour in the chicken stock with a pinch of nutmeg. Season with salt and pepper. Bring to the boil and simmer for 15 minutes.
Purée the soup in a food processor or blender until smooth. Return to a clean pan, stir in the cream and reheat gently. Serve hot.