Rub the pork with salt and pepper. Melt the butter with the oil in a large, flameproof casserole and sear the pork on all sides. Drain to a plate.
Add the mustard seeds and onions and fry for about 5 minutes until beginning to colour. Stir in the garlic and celery and cook for 2 minutes. Add the flour and cook, stirring for 1 minute.
Stir in the thyme, wine and seasoning and let the mixture bubble up. Return the pork to the pan and cover with a lid. Transfer to a preheated oven, 160°C (325°F), Gas Mark 3, for 45 minutes.
Stir the prunes, potatoes and mint into the cooking juices around the pork and return to the oven for a further 1 hour until the potatoes are very tender. Leave to rest for 15 minutes before serving.