First go with a slow cooker, and it tasted pretty good.
3 tbsp olive oil
2 large onions, peeled and very finely sliced
1kg belly pork, skin removed and fat trimmed, cut into 2cm cubes
1 tbsp thyme leaves
1 bay leaf, torn in half
500ml dry cider
1½ tsp English mustard
Switch on the slow cooker, low setting.
Heat 2 tbsp olive oil in a large, ovenproof casserole over a medium-low heat. Add the onions, season well and cook, stirring often until really soft – at least 15 minutes. Don't let them brown.
Meanwhile, heat a frying pan over a medium-high heat with the remaining oil and brown the pork, in batches, seasoning it as you go. It should be nicely caramelised all over. Add the pork, and any juices it has released, to the soft onions, and stir well. Add the thyme and bay leaf and pour in the cider. It should just cover the meat. Bring to the boil then cover the casserole and transfer to the oven. Cook for 7 hours.
Taste the juices, season to taste, stir in the mustard, and serve with plenty of fluffy mashed potato to soak up the juices.
Note: initially I only cooked it for 4 hours, while the meat was cooked through and tasted fine it wasn't as tender as I'd hoped. Another 3 hours sorted that out.