Place the flour, butter and lard in a large bowl. Using your fingertips, rub together until the mixture resembles breadcrumbs. Add about 5 tablespoons ice-cold water and, using your hands, mix to a firm pastry dough. Cover with clingfilm and chill in the fridge for 20 minutes.
Heat the oil in a pan and cook the onion over a medium heat for 6 minutes. Tip into a bowl, then mix in the swede and potato. Season with salt and freshly ground black pepper.
On a lightly floured surface, roll out the pastry dough and, using a 20cm plate, cut out 4 circles. Place a quarter of the vegetable mix on one half of each circle and top with the steak.
Brush the edges with some of the egg. Fold the pastry over the filling and crimp the edges with your fingers. Place on a baking tray lined with baking parchment, brush with more egg, then chill in the fridge for 15 minutes.
Preheat the oven to 200°C, fan 180°C, gas 6.
Bake the pasties for 10 minutes, then reduce the oven temperature to 180°C, fan 160°C, gas 4 and cook for a further 40 minutes.