Season the chicken breasts. Heat the oil in a frying pan over a high heat and brown the chicken very quickly on each side – you might need to fry these in batches. Don’t worry about cooking the chicken all the way through. Set aside.
Set aside 2 tbsp of the stuffing mixture. Make up the rest using half the amount of water that the packet says.
Make the pastry by tipping the flour into a food processor with 1 tsp salt. Add the butter and suet, and whizz to fine crumbs (or rub in with your fingers if you don’t have a food processor). With the motor running (or stir with a fork), dribble in 150ml cold water until the pastry just comes together. Tip onto a lightly floured surface and bring together into a dough. Cover one-third while you roll out the rest to line an oiled, 20cm round springform or loose-bottomed tin.
Spoon the stuffing into the base of the pie, packing down well and smoothing the top. Arrange the chicken pieces over, cutting to fit like a jigsaw puzzle to fill in any gaps. Squeeze sausagemeat from skins and press evenly over the top, then scatter over the reserved dry stuffing. Roll out the reserved pastry until big enough to cover the pie with an overlap. Brush the edge of the pie with egg, then lift on the pastry lid and press edges to seal before trimming and rolling down excess to thoroughly seal.
Decorate by crimping the edges or pressing them with the prongs of a fork, then cut a little hole in the middle of the pastry to let steam escape. Brush all over with more egg (don’t throw the leftover egg away).
Heat oven to 200C/180C fan/gas 6 and bake for 50 mins. Brush all over again with egg and cook for another 10 mins. Cool in the tin, then thickly slice for a picnic or lunch. Or wrap well in cling film and freeze for up to 3 months.