Heat oven to 160C/fan 140C/gas 3. Cut the pork into thick slices, about 3cm thick. Coarsely crush the peppercorns and sprinkle over the pork along with the thyme and some salt and pepper.
Heat 2 tbsp oil in a large flameproof casserole, then add the onions and fry until lightly browned. Add the cabbage and stir well, then add the apples and wine and cook until the cabbage starts to soften. Finally, add the chestnuts, 1 tbsp of the jelly, salt and pepper, and bring to the boil. Cover and simmer for 5 mins.
Meanwhile, heat 1 tbsp of the oil in a frying pan, add the pork and fry on both sides until browned, then stir in the remaining tbsp of the jelly. Cook for a few mins until the pork is deeply browned and glistening. Arrange the pork over the cabbage. Pour a little boiling water into the frying pan, stir well to lift up all the pan juices, then pour over the pork.
Cover the pan tightly, then cook in the oven for 1¼-1½ hrs until the pork is very tender.
Freeze at the end of step 4. Defrost in the fridge overnight. Add a little more water if it looks dry, then bring to the boil. Return to the oven at 160C/fan 140C/gas 3 for 20-25 mins until heated through.