Peel the onion, carrot and garlic. Rinse and trim the celery. Finely dice the onion, carrot and celery. Grate or crush the garlic. Peel the potatoes and chop them. Quarter and core the apples. Thickly slice them (you don't need to peel them).
Strip the rosemary leaves from the stems. Finely chop them. Warm a drizzle of olive oil in a deep frying pan or wok. Add the pork strips and cook over a high heat for 5 mins. Stir every so often till browned.
Fill a kettle and boil it. Pop the pork on a plate. Add the diced onion, carrot and celery to the frying pan. Season. Fry over a medium heat for 5 mins. Stir often. Add the garlic and half the rosemary. Stir and fry for 1 min.
Stir the potatoes and apples into the pan. Add the pork back in. Add the Dijon mustard and 600ml hot water from the kettle to the pan. Season.
Bring to the boil. Cover. Turn the heat down. Simmer for 15 mins or till the potatoes are tender and cooked.
Stir in the crème fraîche. Gently heat till the stew is warmed through and slightly thickened.
Taste and adjust the seasoning if you think it needs it. Ladle the stew into bowls. Scatter with the rest of the rosemary to serve.