Vegan shepherds pie

This recipe is a robust winter warmer and goes great with any plain boiled veg like sprouts, spring greens, cauliflower, carrots, green beans or peas. I usually serve it with a combination of what I fancy veg wise and onion gravy. English mustard goes well with it. When I make this pie I always make more than needed as it can be reheated the next night which saves time cooking or my favourite way of using it up is to put it on toast with margarine spread on it under the grill until golden brown and serve it with tom sauce and a knob of melting margarine on top adding salt and white pepper, my mouth is watering as I write.
And so to the recipe.

generous glug, 4/5 tablespoons sunflower oil
3 medium/large onions diced
2/3 sticks of celery chopped
8oz mushrooms chopped
300gms green lentils boiled until soft
1 teaspoon Natex original yeast extract
salt and pepper
2.5kg potatoes
vegan margarine
a little soya milk

Fry onions, celery and mushrooms in the oil, stirring now and then to prevent burning. This mix must be well fried until it is golden brown to give the rich flavour that makes this taste so good.
Add a little salt ( not too much as the Natex is quite salty) and several good twists of black pepper then add the mixture to the lentils which should be soft and simmering gently. Add Natex and a little water to make a gravy, do not add too much water but you want it to be moist and juicy so add gradually until you have a "creamy" consistency.

Once the veggies are on to fry, peel, chop and boil potatoes with a little salt then mash when cooked with some margarine, 1/2 tablespoons, and a splash of soya milk. I add a little salt and white pepper.
Pour the lentil mixture into a large glass or ceramic baking dish and gently spoon on the mash until the lentils are covered. Level the mash with a fork to leave a ridged appearance which helps it go golden on top and place in the oven for about 1/2 an hour at 200-220 or until it is golden brown