Place the onions, celery and five shredded sage leaves in a slow cooker, then season with salt and freshly ground black pepper. Scatter with the butterbeans.
Season the pork steaks with salt and freshly ground black pepper. Heat the oil in a non-stick frying pan then fry the pork until golden-brown on both sides. Transfer to the slow cooker.
Pour the cider and stock into the frying pan, then bring to the boil. Pour over the pork, put the lid on, then cook on low for seven hours until tender and surrounded with sauce.
Cut the apple into eight wedges and cut away the core. Heat the butter in a frying pan, then add the butter and fry for 8-10 minutes until golden-brown and just tender. Add about 15 more sage leaves, turn up the heat and fry until the leaves are crisp.
Lift the pork steaks from the slow cooker and set aside in a serving dish. Mix the cornflour paste and the mustard into the beans and stir till the sauce thickens a little. Transfer the sauce to the serving dish with the pork, top with the apples and crisp fried sage. Serve with crusty bread and a crisp green salad.