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Heat the oil and butter in an ovenproof casserole over a medium heat. Add the onion, leeks and carrots and a pinch of salt, and fry gently for 10 mins until softened and translucent. Add the chicken, turn up the heat and fry for 5 mins. Pour in the cider, cook for 3 mins, then pour in the chicken stock and add the bunch of thyme. Bring to a simmer, season to taste, then reduce to a low heat and cover the pan. Cook for 30 mins.
Heat the oven to 180C/160C fan/gas 4. Make the dumplings by mixing together the flour, baking powder, mustard powder, the suet or butter, parsley and ½ tsp salt. Swiftly mix in 8-10 tbsp of water and bring together with your hands. The dough should be firm and pliable. Divide into eight equal-sized pieces, weighing for accuracy if you like.
Remove the bunch of thyme from the stew and discard. Spoon a little of the liquid into a bowl and mix with the cornflour, then stir the mixture into the casserole. Add the ham hock, peas and mustard. Arrange the dumplings on top of the stew, then put in the oven, covered, for 30 mins. Remove the lid and cook for a further 10 mins until the dumplings are puffed up and golden brown. Serve in bowls. To freeze the stew, leave to cool completely, then transfer to a freezerproof container. Will keep for up to three months. Defrost thoroughly, then reheat until piping hot.