Make the pastry by rubbing the butter into the flour with a pinch of salt until it resembles breadcrumbs. Add enough cold water to bring it together into a dough. Wrap and chill for 20 minutes.
Preheat the oven to 200C/180C fan. In a bowl, mix the beef mince, diced potato, onion, swede, thyme, salt and pepper.
Roll out the pastry and cut into 4 equal circles about 20cm across.
Divide the filling between the circles, placing it on one half of each. Brush the edge with water, fold over and crimp the edges to seal.
Beat the egg with a splash of water and brush over the pasties. Cut a small slit in the top of each to let steam escape.
Bake for 40-45 minutes until golden brown. Cool on a wire rack for 10 minutes before eating.