Prepare the pie filling: Place the beef and onions in a large lidded saucepan, season with salt, and add enough water to cover. Bring to a boil, cover, reduce the heat, and simmer gently for 2½–3 hours until the meat is tender.

Make the pastry: In a bowl, combine the flour and salt. Add the cubes of margarine and lard, then rub the mixture together with your fingertips until it resembles breadcrumbs. Gradually add water, mixing with a knife until the dough starts to come together. Knead lightly, form into a ball, wrap in clingfilm, and chill for 30 minutes.

Preheat the oven: Set your oven to 200°C/400°F/Gas 6.

Cook the potatoes: Boil the potatoes in salted water until tender, then drain and set aside.

Finish the pie filling: Once the meat is tender, remove the saucepan from the heat. Ladle some of the liquid into a jug, mix with the gravy granules and beef extract, then add to the cooled liquid. Pour this over the meat, heat gently until simmering, and then remove from heat to cool slightly.

Assemble the pie: Roll out a small portion of pastry to 3mm thickness, then cut into strips. Dampen the rim of a 28 x 38cm (11in x 15in) pie dish with water and line with the pastry strips.

Layer the pie filling: Strain the meat and onions, reserving the sauce. Spread the meat and onions evenly in the pie dish, then layer the potatoes on top. Pour over the reserved sauce until just below the top of the potatoes, leaving any extra sauce for serving.

Top the pie: Roll out the remaining pastry to 3mm thickness and cut it to fit the pie dish. Brush the pastry rim with the beaten egg, place the pastry lid on top, seal the edges, and brush the top with the remaining egg. Cut a few slits in the lid to allow steam to escape.

Bake the pie: Cook in the preheated oven for 50–60 minutes, or until the pastry is golden-brown and the filling is bubbling inside.

Serve: Serve the pie hot with the reserved sauce on the side.