Heat the oil in a frying pan over a medium heat and fry the onion until softened, about 6-8 mins. Stir in the garlic and ginger, and cook for 1 min more before stirring in the chicken pieces. Cook for 8-10 mins until the chicken is golden all over. Add the curry paste, stir, then mix in the chopped tomatoes and brown sugar. Cook for 10-12 mins until the liquid has reduced and you have quite a dry curry. Stir in the double cream and cook for 5 mins more until it has reduced slightly – you want a thick sauce, so it doesn’t seep out of the finished pasty. Leave to cool.
To make the pastry, tip the flour into a food processor along with the butter, caraway and cumin seeds if using and pulse to fine crumbs. (You can also do this in a bowl by rubbing everything together using your fingertips.) Mix in the egg yolk and a splash of cold water, if needed,
to bring the mixture into a dough. Wrap and keep chilled for 20 mins while the filling cools.
Heat the oven to 200C/180C fan/gas 6. Roll the chilled pastry out on a lightly floured surface until it’s about ½cm thick. Stamp out discs using a 10-12cm pastry cutter, or use a small plate as a template and cut out with a knife. (You should get about eight in total.) Put a couple of spoonfuls of the filling over each pastry circle, just off-centre, then brush the beaten egg around the filling and fold the pastry over so the edges meet. Crimp the edges or seal with a fork. Put on a baking tray, cut a few small slashes into the top of each pasty, brush over the remaining beaten egg and bake for 20-25 mins until crisp and golden brown. Cool slightly, then serve. Will keep chilled for up to three days.