1. Heat 30ml of the oil in a medium-sized pot and fry the onion, leeks, carrots and celery until glossy (at least 5 minutes, 10 if time allows).
2. In the meantime, remove the sausage casings from the bangers and roll the meat into tiny meatballs. Aim for 6–8 per sausage; this should be the perfect size for the soup.