Heat a large Dutch oven over medium high heat and add olive oil. Once the oil is hot, sear the pork on all sides, working in batches if necessary. Season with salt and pepper. Once the pork is seared, remove it from the pot and hold.
Add the apples, leeks and celery to the pot and sweat for about 5 minutes, stirring occasionally. Season with salt and pepper and add the whole grain mustard, Dijon mustard, sage and rosemary to the pot. Pour the Hard Cider and vegetable stock into the pot. Return pork to pot. Bring the liquid to a simmer and cover the pot. Cook the stew for about 2 hours or until the pork is fork tender.
Remove the sprigs of sage and rosemary. Season the stew with the apple cider vinegar, butter, salt and pepper. Serve hot with grilled bread.