Make the pastry. Add flour, turmeric, a little salt and the fat to a mixing bowl, and rub the fat in with your fingertips or using an electric mixer until you have fine crumbs. Gradually mix in enough of the measurement water to form a soft but not sticky dough. Knead lightly, then wrap in clingfilm and chill while making the filling.
Heat the oil in a frying pan, add the chicken and butternut squash and fry for 5 minutes until the chicken is just beginning to brown. Add the onion, garlic, chilli and pepper and fry for 5 minutes until the vegetables are softened and the chicken cooked through. Add the curry powder, coriander and a little pepper and cook briefly, then take off the heat and leave to cool.
Cut the pastry into 4 pieces, roll each piece out on a lightly floured surface and trim to an 18 cm (7 inch) circle. Divide the filling between the pastry circles, brush the edges with beaten egg, then fold in half and press the edges together well, first with your fingertips, then with the prongs of a fork, until well sealed.
Transfer to an oiled baking sheet, brush the patties with beaten egg and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes. Serve hot or cold with chilli tomato chutney.