1. Pre-heat the oven to 180 °C.
2. In a mixing bowl, combine the roast chicken, mayonnaise, mustard and grated cheese.
3. Very finely slice the white and pale-green parts of the spring onions, add them to the bowl and mix well. Season to taste with salt and black pepper.
4. Lay a long piece of clingfilm on your kitchen counter and lightly dust it with flour. Unroll the first pastry roll on the clingfilm, with the long side facing you.
5. Using a ruler and the tip of a sharp knife, cut the pastry into four equal rectangles.
6. Place an eighth of the filling on one half of the first rectangle, piling it up in the middle and leaving an open border 1½ cm wide around the pile.
7. Lightly beat the egg and use a pastry brush to ‘paint’ egg all the way round the edges of the rectangle.
8. Fold the other half of the rectangle over the filling and press down lightly on the open edges to form a neat parcel.
9. Firmly press the back of the fork all the way round the three open edges of the pie, to seal. If the fork sticks to the pastry, dip the prongs in a little flour.
10. Cut a small slit in the top of the pie and place on a baking sheet. Repeat these steps with the remaining three rectangles, and with the second sheet of pastry, to make 8 pies.
11. Brush more beaten egg over the pies and bake at 180 °C for 25-30 minutes, or until golden brown and crisp.