Step 1

To make the pastry, put the flour, salt and butter in a large bowl. Rub the butter gently through the flour, make sure that chunks of butter remain in the mixture. Gradually add the cold water and lemon juice. Use your hands to bring the ingredients together into a rough dough.

Step 2

Tip the dough out onto a lightly floured work surface and form into a scraggy ball. Chill in the fridge for 15 minutes.

Step 3

On a lightly floured work surface, roll out the pastry into a rectangle that is about 1cm/½in thick and three times as long as it is wide. Brush any excess flour off. Fold the top third of the pastry down and the bottom third up over that to create a block. Turn 90 degrees and roll out to the same size rectangle as before.

Step 4

Fold into a block as before. Wrap in baking paper and chill for 15 minutes.

Step 5

Remove from the fridge and repeat the rolling and folding twice (so it has been rolled and folded four times all together). Wrap in baking paper and chill for 30 minutes before using.

Step 6

To make the filling, melt the butter in a large frying pan over a medium heat. Fry the onion for a few minutes until soft and translucent.

Step 7

Sprinkle in the flour and mix well. Cook for 2 minutes, stirring constantly. Gradually add the stock and simmer until the sauce thickens slightly. Add the mustard and cream and stir. If the sauce is too thick, add a splash of stock or water.

Step 8

Take the sauce off the heat, add the leftover meat and vegetables and stir well. Season with salt and pepper, then set aside to cool completely.

Step 9

Preheat the oven to 200C/180C Fan/Gas Take the pastry from the fridge and cut off about a third of the pastry to use as the lid.

Step 10

Roll out the larger piece of pastry to about 5mm thick on a lightly floured surface and cut out a circle (or oval or rectangle) that fits into a 1 litre/1¾ pint pie dish.

Step 11

Line the pie dish with the pastry and fill with the pie mixture. Brush the pastry edges with the beaten egg.

Step 12

Roll out the remaining pastry to make a lid and lay over the filling, crimping the edges with your fingers to seal. Trim away any excess pastry. Brush the top all over with the egg wash and set aside in the fridge for 10 minutes (or even overnight) to firm up.

Step 13

Bake for 35–45 minutes. When the pastry is golden-brown and crisp all over, remove from the oven and allow to cool for 10 minutes, then cut into slices and serve.