In a large saucepan, fry the onions and garlic in the oil over a medium heat for about 4 minutes. Add the mushrooms and toss well. Add chunks or cubed burgers and the thyme. Cook for 5 minutes over a low heat. Add the gravy and simmer for 15-20 minutes until it has thickened. Add the soy sauce and vinegar and season to taste with salt and pepper. Set aside to cool.

Place a china pastry support in the centre of a large pie dish, then spoon in the filling.

Roll out the pastry 5 millimeters / 1/4 inch thick to the shape of the dish. Cut a strip 2 centimeters wide from around the edge of the pastry. Continue rolling the main piece of pastry until it fits the dish once more. Dampen the rim of the dish with water and place the pastry strip around it. Moisten the attached pastry strip, then cover with the pastry lid. Press the edges together firmly and trim the excess pastry. Use the back of a knife to make a decorative edge. You can re-roll the trimmings to make decorative leaves. Make a small round hole in the centre of the pie. Brush the pastry lid with soy milk.

Bake the pie in the oven at 230C/445F until the pastry has risen and turned lightly golden, about 10-12 minutes. Reduce the heat to 200C/390F and cook for a further 20-25 minutes until crisp and golden brown. Serve at once with more gravy and some mashed potatoes.