Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion, celery and carrot and fry for about 5 minutes or until softened.
Add the sugar, vinegar, fish stock, bay leaves, cloves and a little salt and pepper and bring to the boil. Arrange the beetroot in the base of the slow cooker pot, then top with the apples. Slash the fish two or three times on each side, then arrange in a single layer on top of the apples.
Pour over the hot stock and vegetables, cover with the lid and cook on high for 1½–2 hours or until the fish flakes easily when pressed in the centre with a knife.
When almost ready to serve, mix together the ingredients for the mustard cream in a small bowl. Carefully transfer the fish, vegetables and some of the stock to shallow bowls, then garnish with chives. Serve with spoonfuls of the mustard cream and crusty sliced bread.