Grease a 20cm/8in springform cake tin with vegetable fat.
For the filling, put the coriander seeds in a small dry frying pan set over a low heat. Keep on a low heat and toast lightly until they just start to change colour. Once done, put the coriander seeds in a pestle and mortar and crush them to a powder.
Heat the oil in a large frying pan. Add the fenugreek seeds and the whole chillies and stir until they turn golden-brown. Add the onions and cook until browned. Stir in the garlic, ginger, crushed coriander seeds, chilli powder and ground coriander.
Add the chicken and fry over high high for five minutes, or until the chicken starts to colour. Stir in the chopped tomatoes, cook for five minutes then add the salt, mango powder and garam masala. Cook for 15 minutes, or until the chicken is cooked through. Transfer to the fridge to cool. Remove the whole chillies and discard.
Preheat the oven to 200C/400F/Gas 6.
To make the pastry, combine the flours in a large bowl. Add the butter and, using your fingertips, rub it into the flour until it resembles breadcrumbs. In a pan, heat 200ml/7fl oz water, the salt and fat until just boiling. Pour the liquid onto the flour mixture and mix using a wooden spoon.
Tip the dough out onto a lightly floured work surface and knead to a smooth dough.
Working as quickly as possible, take two thirds of the pastry and roll out a large circle of dough on a lightly floured surface. Using the rolling pin to help you, line the prepared tin.
Spoon the filling into the pastry lined tin. Press it down and level the surface.
Using the remaining pastry, roll out a pie lid on a lightly floured work surface.
Brush the top edges of the pastry in the tin with beaten egg yolk and place the pastry lid on top. Crimp the edges to seal and trim off any excess pastry. Make a couple of slits in the top of the pie to allow the steam to escape and brush the pastry lid with egg yolk. Use any leftover pastry to make decorations for the top of the pie.
Bake for 1 hour, or until the top is golden-brown. Leave to cool in the tin for 10 minutes then remove from the tin and leave to cool completely on a wire rack before serving.