Melt the butter in a large saucepan, add the parsnips and cook, without browning, over medium heat for 10 minutes. Add the stock, salt, pepper and grated horseradish (if using fresh), and bring to the boil. Reduce the heat and simmer the soup for 20 minutes, or until the parsnips are tender and cooked through.
Remove the saucepan from the heat and leave to cool for 5 minutes. Purée the parsnips with the stock, in batches, in a food processor or blender. Return the soup to the saucepan and, if using horseradish sauce, stir it into the soup now. Stir in the cream and reheat the soup, stirring occasionally, for 5–10 minutes.
Ladle the soup into four warmed soup bowls or mugs, and garnish with a little grated nutmeg and the sprigs of parsley.