Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter in a large frying pan, add the onion, potatoes and bacon and fry gently, stirring, until just beginning to colour.
Transfer the potato mixture to the slow cooker pot. Pour over the boiling stock, then add the sweetcorn, bay leaf and a little salt and pepper. Cover with the lid and cook on high for 2–3 hours or until the potatoes are tender.
Add the fish and press it just below the surface of the stock, cutting the pieces in half, if needed. Replace the lid and cook, still on high, for 30 minutes or until the fish flakes easily when pressed in the centre with a knife.
Lift the fish on to a plate with a fish slice and break it into flakes with a knife and fork, checking for and removing any bones. Stir the cream into the soup, then return the fish. Ladle the soup into bowls and sprinkle with parsley.