Cut the fat into the flour and salt in a mixing bowl. Rub with your fingertips until the mixture resembles breadcrumbs. Stir in a very little cold water – if you have any iced water in the fridge all the better – until the mixture only just begins to stick together. Gather up into a ball and leave in the fridge for at least half an hour, or until the next day if you like.
Heat oven to Gas 4 (350°F/180°C). Melt a little olive oil and 1–2 oz butter in an iron casserole dish on top of the stove and remove from the heat. (If you haven’t got an iron casserole do it in a large frying pan and transfer later to an ordinary casserole dish.) Cut the chicken up into fairly small pieces. Stir the chicken and the sliced onions and carrots into the melted fat in the casserole. Stir in the flour. Meanwhile heat the cider and stir it into the chicken mixture. Add salt and pepper, the coriander and the mustard. Bring to the boil, stirring once or twice and when it thickens and is simmering cover the dish and cook in the centre of the oven at Gas 4 (350°F/180°C) for ¾ hour, stirring it once during the cooking. Take out and add the chopped apple and sliced mushrooms and then return the covered dish to the oven for a further 10 minutes. It should be a thick and creamy casserole – if it looks too thin at the end blend a little more flour with some water and add it to the stew. Then bring to the boil and simmer, stirring, for 3 minutes. Stir the soured cream into the casserole, check for seasoning and pour the mixture into a large shallow dish – I use a china flan dish about 9–10 in (23–25.5 cm) diameter, 1½ in (4 cm) deep. Let cool. Roll out the pastry fairly thickly, roughly the size and shape of your dish. Dampen the edges of the dish. Roll the pastry back over the rolling pin and out again on to the dish. Trim the edges of the pastry very slightly larger than the dish and then press down the edges lightly. Roll out the scraps of pastry and use to decorate as you like. For a special occasion the pie will look really professional and beautiful if you take time doing the decoration and glaze it by brushing all over with an egg yolk. Heat the oven to Gas 6 (400°F/200°C) and bake just above the centre for 25–30 minutes until golden brown.