Put the flour and butter in a bowl. Rub together until you have a crumb. You can do this in the food processor, if you prefer.
Add the water and cut through with a cutlery knife (or pulse in the food processor) until evenly distributed - the dough should be clumpy. If it’s too dry to gather, you can add a tiny bit more water but be very sparing.
Gather into a smooth ball, wrap and refrigerate until 15 mins before you’re ready to use.
Make the filling
Melt the butter in a large pan over a low heat.
Add the leeks and garlic and sweat for about 2 minutes until soft.
Add the vegetarian chicken-style pieces. Fry for about 5 minutes until the leeks just start colouring.
Stir in the peas, then set aside to cool.
Make the sauce
Put the whole milk, butter and flour in a pan.
Whisk over a low heat until it thickens.
Taste to make sure the flour has cooked out, season and set aside to cool.
Bring it all together
Preheat the oven to 200C (180C fan, 400F).
Spoon the cooled filling into a small greased roasting tray.
Pour the creamy sauce on top.
Roll out the pastry to a bit bigger than the top of the dish and lay it on top.
Press with a fork all the way around. The excess pastry should fall away and any left can be trimmed with a sharp knife.
Brush the top with milk to give a little shine to the finished pastry.
Cut a couple of slits in the top of the pie to let steam escape.
Put the pie in the oven and bake for 20-25 mins until golden on top.