Preheat the slow cooker if necessary; see the manufacturer's instructions. Bring a large saucepan of water to the boil, add the potatoes and cook for 3 minutes, then drain.
Heat the butter and oil in a frying pan, add the onions and bacon and fry, stirring, for 5 minutes or until just beginning to turn golden. Stir in the apple and flour and season the mixture well.
Layer the potatoes and the onion mixture alternately in the slow cooker pot, ending with a layer of potatoes. Bring the stock and mustard to the boil in the frying pan, then pour into the slow cooker pot and add the bay leaves. Cover with the lid and cook on low for 9–10 hours.
Sprinkle the top of the potatoes with the cheese, lift the pot out of the housing using oven gloves and brown under the grill, if liked, then spoon into shallow bowls. Serve with grilled tomato halves sprinkled with chopped chives, if liked.