Start by making the pastry. Place the plain flour and salt in a large bowl and whisk to mix, or place in a food processor and pulse until combined.
Add the cold diced vegan block butter and rub it in using your fingers or blend with the food processor until the mixture resembles fine breadcrumbs and no lumps of fat remain.
Add the cold vodka (or water) and mix; drizzle in a little ice cold water as needed to bring the pastry together into a ball.
Divide the pastry in two, one piece about two thirds of the pastry and the other one third. Shape each piece into a disc. Cover the pastry discs and pop them in the fridge for at least half an hour until it has firmed up a bit. (The pastry can be kept in the fridge for up to two days).
To make the filling, gently heat the butter in a large pan. Add the onion and leeks and cook gently, stirring often, for about 10-15 minutes until they are soft.
Add the crushed garlic, sliced mushrooms, bay leaves and thyme and cook over a medium heat until the excess liquid has evaporated. Add the white wine and cook until it has evaporated.
Sprinkle over the flour, stir well and cook for a minute then gradually stir in the stock, a little at a time.
Stir in the creme fraiche or cream, wholegrain mustard, chickpeas and plenty of salt and pepper. Simmer over a low heat for about 15 minutes, stirring often. If it is too dry you can add a splash more vegetable stock but don't add too much, if the filling is too wet it will make the pastry soggy.
Remove from the heat, allow to cool then cover and refrigerate until cold. It can be kept covered in the fridge for up to two days.
To assemble the pie, roll out the larger piece of pastry on a floured surface until it is large enough to line a 23 cm / 9 inch round 5 cm / 2 inch deep pie dish. (If the pastry has become too firm to roll out, let it come up to room temperature for half an hour first).
Press the pastry into the corners and up the sides and trim off the excess, leaving about 3 cm / 1 inch overhanging. Reserve the trimmings.
Spoon the cooled filling into the pie dish and spread it level.
Roll out the second piece of pastry into a circle large enough to top the pie. Brush the overhanging edge of the pastry base with water then place the circle of pastry on top and press the edges together to seal.
Trim off the excess then crimp the edge of the pastry. Cut a couple of slits in the middle of the pie to allow steam to escape.
Bring together the pastry trimmings, roll them out again and cut out decorations for the top of the pie, using a dab of water to stick them on.
Place the pie in the fridge for 20 minutes while you preheat the oven to 200°C/180°C fan/400°F/gas mark 6.
Brush the top of the pie with non-dairy milk then bake for 50 minutes until golden and the filling is bubbling. If it starts to get too dark before the baking time is up, loosely cover the top with tin foil.
Leave the baked pie to sit for 15-30 minutes before slicing it.