Heat the oil in a large frying pan. Add the onion and garlic and cook for 2 minutes. Add the apples and cook, turning occasionally, for 2 minutes until golden.
Stir in the pork and sprinkle over the flour. Stir gently to combine before pouring in the chicken stock and mustard. Bring to a simmer and cook gently for 15 minutes. Give it a good stir, cover with a lid and cook for a further 15 minutes. Stir in the chopped watercress just before serving.
Meanwhile, cook the mashed potato to pack instructions. Serve the pork and apple with the mash and remaining watercress.