Preheat your oven to 190°C / 375°F / Gas Mark 5. Prick the potatoes a couple of times, spray with a little cooking spray or rub with oil, sprinkle with salt, then bake for 1 hour or until a knife passes easily into the centre. When cooked, remove from the oven and set aside to cool.
Meanwhile, heat the oil in a large lidded frying pan or casserole dish, and fry the onion and celery for 3-4 minutes until soft. Add the carrot and parsnip and fry for a further 3-4 minutes, and then the mushrooms. Continue to cook gently for 5 minutes until the mushrooms have released most of their liquid.
Add the puy lentils, tomato purée, wine, stock powder and water to the pan, then cover and simmer for 30 minutes or until the lentils are tender.
When the potatoes are cool enough to handle, slice off the top of each (about two thirds of the way up the potato). Carefully scoop out the flesh with a spoon, leaving just enough around the skin to keep the shape of the potato. Place the flesh into a large bowl, add the margarine and mash thoroughly until no lumps remain. Season with salt and pepper to taste.
Divide the pie filing between the four potatoes, and use a piping bag with a large nozzle to add the mashed potato topping to the pies. (If you a short of time, just dollop it on and spread with a knife instead!).
Return the potatoes to the oven for 10-15 minutes until heated through, and the potato topping is turning golden brown.