Heat the oil in an ovenproof casserole over a medium heat. Add the onion and leeks with a pinch of salt, and fry gently for 5 minutes until softened and translucent.
Now add the chicken and gammon, turn up the heat and fry for 5 minutes, stirring occasionally. Add the flour and cook for 1 to 2 minutes. Pour in the wine, cook for 3 minutes, then pour in the 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 1 hour. After this time, stir through the cider vinegar, Dijon mustard and cream. Season and remove the thyme.
Preheat the oven to 180C/160 fan/gas 4. Make the dumplings by mixing together the flour, baking powder, suet, parsley and salt. Combine the buttermilk, mustard and water in a jug and then swiftly mix into the dry ingredients. Bring together with your hands – the dough should be firm but pliable. Form the mixture into 8 equal dumplings.
Arrange the dumplings on top of the stew, then cook in the oven, covered, for 30 minutes. Remove the lid, spoon in the peas around the dumplings, and cook for a further 15 to 20 minutes until the dumplings are puffed up and have a golden brown crust. Serve in bowls.