Rub the butter into the flour and salt until it resembles breadcrumbs, add enough cold water to bring it together into a firm dough, wrap and chill for 30 minutes.
Toss the beef, potato, swede and onion together in a bowl with a good pinch of salt and a very generous amount of black pepper. The filling goes in raw and cooks in its own steam.
Divide the dough into 4 and roll each piece into a round about 22 cm across. Pile a quarter of the filling down the middle of each round, leaving a 2 cm rim clear.
Wet the rim, fold the pastry over into a half moon and crimp the edge by folding it over on itself in small tight twists all the way along, working to one thick seam.
Brush with beaten egg and bake at 200C (180C fan) for 20 minutes, then turn the oven down to 170C (150C fan) for another 20 to 25 minutes until deep golden. Rest for 10 minutes before eating, as the inside is molten straight from the oven.