Heat the vegan butter in a wide pan over medium heat, then add the sliced mushrooms. Fry for 5 minutes until slightly softened and browned, then add the leek and garlic.
Fry for a further 5 minutes until the leek has softened and cooked down. Stir in the dried herbs.
Add the flour to the pan, and mix it evenly through the vegetables. Pour in the milk gradually, stirring as you go, until no flour lumps remain.
Stir for 2-3 minutes until the mixture thickens. Add the nutritional yeast, wholegrain mustard and miso, and stir to combine. Season to taste with salt and ground black pepper then set to one side to cool*.
Preheat the oven to 200°c/390°f.
Once the filling has cooled, spoon it into 2 individual pie dishes. Cut out two circles from the pastry, large enough to cover the mushroom filling and the edge of the dish. Use a little of the remaining dough to cut or roll two long thin strips, long enough to go all the way around the dishes.
Brush a little milk onto the edges of the dishes, then stick the strips all the way around. Brush a little milk onto the strips of pastry, then stick the disks to the top, pressing down and using a fork to crimp all the way around.
If you would like to, you can use milk to stick some decoration to the top of the pies. Otherwise, simply use a knife to poke a few holes for steam to escape.
Brush the tops of the pies with a little plant milk. Place the pies in the preheated oven, and cook for 30 minutes, or until golden and crisp on top.