1Preheat the oven to 200°C if fan-forced (220°C if conventional).
2Heat the olive oil in a large frying pan over medium heat and sauté the garlic for 30 seconds, until aromatic. Add the leek, mushrooms and spring onions.
3Crumble the stock cube over the veggies, then stir frequently over medium heat for 5 minutes or so, until the liquid has reduced. Mix in the pepper and salt to taste. Set aside.
4To make the white sauce, melt margarine in a large saucepan over medium heat. Add the flour and stir for about 1 minute, thoroughly blending the mixture. Gradually add the soy milk, mixing in one splash at a time until you have a thick sauce. Once a thick gravy consistency is reached, and the mushroom and leek mixture and stir to combine. Remove from the heat and set aside.
5About 10 minutes before you plan to use them, take the puff pastry sheets out of the freezer and separate them. Once they have thawed, but while they are still cold, use them to line the base and sides of a large rectangular baking dish, measuring about 40 x 30 cm and about 3 cm deep, leaving enough pastry aside to cover the top of the pie; you will probably need to cut the pastry to fit the dish. Transfer it to the oven and bake for 5-10 minutes, until the pastry is light golden brown.
6Remove the baking dish from the oven. And the mushroom mixture, then lay a final sheet of puff pastry over the top. Return to the oven and bake for a further 15 to 25 minutes, until the pastry is puffed, cooked through and golden brown.
7Remove from the oven and allow to cool for 5 minutes before serving.