Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter in a large frying pan, add the white leek slices and fry for 2–3 minutes until softened. Stir in the root vegetables and celery and fry for 4–5 minutes.
Add the pearl barley to the slow cooker pot. Then add the fried vegetables, boiling stock and sage. Stir in the mustard and a little salt and pepper. Cover with the lid and cook on low for 8–10 hours or until the vegetables and barley are tender.
Make the dumplings. Put the flour, suet, bacon and a little salt and pepper in a bowl and mix well. Gradually stir in enough water to make a soft but not sticky dough. Knead lightly on a floured surface, then shape into 12 balls.
Stir the reserved green leek slices into the soup, add the dumplings, spacing them slightly apart, then replace the lid and cook, still on low, for 45-60 minutes or until light and fluffy. Ladle into bowls and serve.