Heat the butter in a large saucepan, add the onion and white diced leeks and fry for 5 minutes until just beginning to soften. Add the other diced vegetables and sage, toss in the butter then cover and fry for 10 minutes, stirring occasionally.
Pour on the stock, season with salt and pepper and bring to the boil. Cover and simmer for 45 minutes, stirring occasionally until the vegetables are tender. Remove the sage then taste and adjust the seasoning if needed.
Make the dumplings by mixing the flour, mustard powder, sage, suet and bacon in a bowl with a little salt and pepper. Gradually stir in the water and mix first with a spoon, then squeeze together with your hands to make a smooth dough. Cut into 18 slices and roll each slice into a into small ball.
Stir the remaining green diced leek into the soup. Add the dumplings to the simmering soup, re-cover the pan and cook for 10 minutes until the dumplings are light and fluffy. Ladle into bowls and serve immediately.