Spoon the borlotti beans into a lidded ovenproof dish and squash with the back of a spoon to loosen the skins. Add the celery, spring onion, bacon pieces and garlic. Stir in the chilli flakes and the tomato purée.
Pour over the water, add the stock powder or bouillon and stir again until well combined. Cover the dish with the lid.
Transfer the dish to the oven and cook for 25-30 minutes, or until the soup has thickened. Stir well before serving in soup bowls with bread and butter on the side, if desired.