Melt the butter in a casserole dish set over a medium heat. Add the celery and turnip and sweat for 15 minutes, stirring occasionally. Add the stock after 15 minutes and bring to the boil.
Reduce to a simmer and cook gently for another 15 minutes until the turnip is soft. Remove from the heat and purée using a stick blender, making sure you leave some texture. Season to taste and garnish with the chopped parsley before serving.