For the pastry, blend the flour, baking powder, salt, butter and egg yolks in a food processor until the mixture resembles breadcrumbs.
With the motor still running, gradually add the water in a thin stream until the mixture comes together as a dough. (NB: You may not need to use all of the water.)
Wrap the pastry in cling film and chill in the fridge for one hour.
Meanwhile, for the filling, preheat the oven to 180C/350F/Gas 4.
Place the chopped beef and flour in a bowl and mix until the meat is coated in the flour. Season, to taste, with salt and freshly ground black pepper.
Season all of the vegetables, to taste, with salt and freshly ground black pepper.
When the pastry has chilled, divide it into six equal pieces. Roll out each piece of pastry onto a lightly floured work surface until it is just larger than the diameter of a dinner plate. Make sure not to tear the pastry.
Using a dinner plate as a template, trim the edges of each piece of pastry to form a disc.
Spoon some of the chopped potato onto one half of each pastry disc, leaving 1.5cm/¾in free at the edges.
Spoon some of the chopped swede on top of the layer of potato, then spoon some of the chopped onion on top of the layer of swede. Finally, spoon over some of the beef.
Brush the edges of the pastry discs lightly all over with a little of the beaten egg.
Cover the filled side of each pastry disc with the other half of the pastry disc and press the edges together with a fork to seal.
Brush the top of each pasty with more beaten egg. Using a sharp knife, cut a small cross shape in the top of the pasties through which the steam can escape.
Place the pasties onto a baking tray and transfer to the oven. Bake for 45-50 minutes, or until the pastry is crisp and golden-brown and the filling is cooked through and piping hot (use a meat thermometer to check trhe temperature of the interior).
When the pasties are cooked, remove them from the oven and set aside to rest for 5-10 minutes, then serve.