Simmer that celeriac in a large pan until soft enough to mash, then do so with salt and a good helping of butter. You'll probably want to give it 20 - 30 minutes to cook through, depending on the size of the chunks.
In another large pan with some oil, saute the mushrooms with a little salt until they start to cook through and some of the excess water cooks off, then set them aside with the pork.
Cook the mirepoix of carrot, celery and onion, and the garlic, on a low-medium temperature with some salt. Stir frequently to cook it all through evenly.
Add the pork and mushrooms back in, then the mixture of stock and cider. Add a bay leaf and some peppercorns, then simmer for about 2.5 hours or until the sauce is well reduced and the meat tender. If the meat isn't tender by the time the sauce thickens, top up the liquid with a little water.
When it's nearly ready, add some honey and some mustard. You can also add some cider vinegar if desired. Then take it off the heat, stir in a little salt and butter, then serve with some fresh parsley.