Lay the thickly sliced onions in a roasting tin and pour in the white wine. Put the pork on top and roast for 30 minutes. Turn down the oven to 160c/fan 140c/gas 3 and continue cooking for 3 hours.
Lift the pork off the onion and let it rest under foil and a tea towel. Pour off most of the fat from the pan, then stir in the flour and cook it for a couple of minutes on the hob. Add the chicken stock and bubble everything together to make a gravy, strain into a jug and serve with the pork.