Preheat an oven to 200°C/400°F/gas mark 6. Melt 50 g of margarine in a large non-stick frying pan. Roughly chop the bottom 7 cm of the spring onions and sauté briefly. Save the tops as you will need them later.
Cut the mushrooms into 5 mm slices and add to the pan. Continue to sauté for 5 minutes until starting to colour, stirring to prevent sticking.
Remove from the heat and sprinkle with the cornflour. Once back on the hob, add the yeast extract and ale. Cook for a further 5 minutes or until mushrooms begin to soften. Use more ale if the mixture seems too dry.
Divide the mixture into four small soufflé/ceramic pie dishes, then cut out four pastry circles to place on top, leaving some room to rise. Brush with soya milk and make a small hole in the top to let the steam escape. Bake for 20 minutes until golden.
Cut the potatoes into large chunks and boil until soft but not falling apart. Drain and put back into the pan to dry slightly. Add the remaining 50 g margarine and mash well. Finely chop the green tops of the spring onions and add, with enough soya milk, to make a smooth mash.