Make the pie filling: melt the butter in a pan over medium heat, then add the mushrooms, leek and garlic and season well. Cook until the mushrooms give up their liquid and are tender.
Sprinkle with the flour, stir well to coat and cook the flour a bit (about 1 minute).
Add the stock, milk, thyme and cooked meat ensuring the meat is coated in sauce and the sauce is thickened and not lumpy. Taste and adjust seasoning. Set aside to cool down a bit.
Pre heat your oven to 190 C / Gas 5.
Cut your pastry lid and set aside. Line the rim of your selected pie dish with a thin strip of pastry (use trimmings to be frugal). Fill your pie dish with the filling. Fix on the lid, put some air vents in it. Glaze with a little milk or egg wash.
Bake for 25 to 30 minutes until golden brown. Serve it with whatever veg takes your fancy.
This is what I make for a pie for 2 people. Increase your quantities if you have more mouths to feed!