Preheat the oven to fan 120C/conventional 140C/gas 1. Cut each leek into about five pieces, chop the carrots into pieces the same size as the leeks. Heat a large, lidded, flameproof casserole dish on the hob until it’s very hot. Add 2 tbsp of the olive oil, then carefully tip the pork into the casserole and leave it for a couple of minutes to brown. Stir once, then leave for another couple of minutes. Using a slotted spoon, transfer the meat to a plate. Pour the rest of the oil into the dish, tip in the leeks, carrots and celeriac and fry for 3-4 minutes, stirring, until they start to brown. Add the garlic and fry for a minute more.
Stir the pork and any juices into the vegetables, then pour in the wine, stock, orange juice and soy sauce. Throw in the rosemary and orange zest, season with salt and pepper, give it a stir, then bring everything to the boil.
Cover the dish, transfer it to the oven and cook for 2 hours, stirring after an hour. Cook until the pork is very tender and the leeks fall apart when prodded with a spoon. (It can now be left to cool and then frozen for up to 1 month.) Leave to stand for at least 10 minutes, then spoon into bowls. Serve with crusty bread to soak up all those juices.