To make the pastry, tip the flour into a bowl with ½ tsp salt. Pour 200ml water into a saucepan and bring up to the boil, then take off the heat, add the lard and leave to melt. Pour the mixture into the flour, beat with a wooden spoon and work into a dough. Cut a third of the pastry off for the top and wrap in cling film then, while still warm, roll out the other two-thirds of the pastry to the thickness of a £1 coin. Butter a 20cm springform cake tin and line with the pastry, leaving a good amount of excess hanging over the side.
Heat oven to 200C/180C fan/gas 6 with a sturdy baking tray in it. Make the filling by mixing all the ingredients, except the cheese, together in a bowl and season generously with cracked pepper and a little salt. Pack half the filling well into the lined tin, cover with a layer of cheese slices, then pack over the rest of the filling. Roll the remaining pastry out and lay on top of the filled pie tin, trim the edges leaving a little overhang, and save the trimmings. Crimp the pie all the way around the edge, then brush the top with the beaten egg. Score the top of the pie with the back of a knife to decorate then, using a small cutter, press a hole into the middle of the pie to let steam escape.
Bake for 15 mins, then reduce the heat to 180C/160C fan/gas 4 and cook for a further 50 mins-1 hr until golden brown. Leave until cold. Can be made the day before and kept at room temperature or chilled for two days. Wrap the pie well and take to the picnic in the tin, with a board and a large knife for serving.