Heat the oven to 200°C.
Cut about 7cm off the bottom part of the spring onions and chop roughly.
Heat 50g of margarine in a large non-stick frying pan. Once melted, add the spring onions and sauté quickly.
Slice the mushrooms into 5mm pieces then throw into the pan. Sauté for about 5 minutes until colouring, while stirring constantly.
Remove from the heat then sprinkle with the cornflour. Put back over the heat and pour in the Marmite and 300ml of ale. Cook until the mushrooms are tender (about 5 minutes). You can add more ale if needed.
Pour evenly into 4 small soufflé/ceramic pie dishes. Roll out the pastry then cut 4 circles the size of the dishes and lay them over the mixture. Using a brush, spread the soy milk on top then cut a small hole in the center.
Put in the oven for about 20 minutes, until well-risen and golden.
Cut the potatoes into large pieces. Fill a saucepan with water, sprinkle with salt then bring to the boil. Add the potatoes and boil until they become tender. Strain in a colander then put back into the pan to dry a bit more. Mix in the rest of the margarine and purée. Thinly dice the remaining green part of the spring onions. Stir into the mashed potatoes then pour enough soya milk to make a creamy purée. Serve with the ale and mushroom pies.