Rub the butter and lard into the flour and salt until the mixture resembles breadcrumbs. Add the water gradually and knead into a smooth dough. Wrap and rest for 30 minutes.
Heat the oven to 220C (200C fan, gas mark 7) and line a baking tray with baking paper.
Divide the dough into four and roll each piece into a 20cm round. Layer potato, swede, onion and beef down the centre, seasoning generously with salt and pepper, and dot with butter.
Brush the edge with water, fold over to form a half-moon and crimp the edge by folding it over itself in small pleats. Cut a steam hole and brush with beaten egg.
Bake for 20 minutes, then reduce the oven to 180C (160C fan, gas mark 4) and bake for 35 to 40 minutes until deep golden.
Rest for 10 minutes before serving hot, or leave to cool completely for a packed lunch.