Start by boiling the potatoes until tender. Drain and mash with the margarine, soya milk, salt and mustard. Put the mash to one side.
Now prepare the filling.
In a pan, dry fry the tofu and then marinade in the soya sauce, water and olive oil. Marinade for at least 15 minutes.
Next, In a large frying pan, melt 1 tablespoon of margarine over a low heat and add the leaks. Cook the leeks, stirring occasionally , for about 18-20 minutes until tender but not browned.
Whilst the leeks are cooking, prepare the sauce.
Mix together the stock, wine and soya milk in a jug.
Place the 50g margarine and 50g flour in a saucepan over a low heat and combine to make a thick roux. Cook for about 30 seconds to be sure that the flour is cooked out and when you have a thick paste, gradually add the liquid, whisking continuously until you you have a smooth sauce.
When smooth add the parsley, tarragon, cornichons, capers, margarine, peas, agave and dijon mustard.
Pour in the leeks, add the drained tofu, and stir together gently to combine all the ingredients.
Pour the mixture into a pie dish.
Top with the potato and bake in the centre of a preheated oven at 200 degrees for 40-45 minutes until the topping is golden brown. If you wish you can grate some vegan cheese over the topping, but this is optional.
Serve hot with a crisp salad or seasonal vegetables.