Line a baking tray with parchment paper. Preheat oven to 180°C/350°F/Gas 4.
Firstly make the pastry: rub the butter into the flour, mustard powder and salt with your finger tips until the mixture resembles fine breadcrumbs. Add the egg and mix well, then add enough milk to make a firm dough. Wrap the pastry in cling film and leave to rest in the fridge.
For the filling, melt the butter in a pan, add the onion and sage and cook, stirring regularly until the onion is soft – this will take about 5 minutes. Season and leave to cool.
Mix the Golden Turkey, ham, onion mixture, Gruyere cheese, parsley, cream, mayonnaise and Dijon mustard and season with a little sea salt and black pepper. If the mixture is a little dry, add more cream.
Divide the pastry into 6 equal pieces and roll each out to a 23cm disc on a lightly floured board. Lay the pastry discs out and divide the filling equally in the centre of each disc, leaving a margin at the edge.
Brush the pastry around the edge with the beaten egg and carefully draw the edges together to form a seam across the top, pinch together to seal. Repeat with the remaining pasties.
Place the pasties on a baking tray, glaze with the egg and bake for 10 minutes then reduce the oven temperature to 170°C/325°F/Gas 3 and cook for a further 15-20 minutes, until golden brown.
Serve hot or cold.
Recipe courtesy of Golden Turkeys