Place a large pan of water on to boil. Add the cubed parsnip and cook until soft enough to mash. Mash with a knob of butter and set aside.
Preheat oven to Gas 4, 180°C, fan 160°C.
Heat the oil and fry the onion and garlic until soft.
Grind the cashew nuts in a nut mill, blender or mincer, then mix with the breadcrumbs in a large mixing bowl.
Beat the egg and add it to the dry ingredients, then mix in the mashed parsnips and herbs.
Add the fried onion, being sure to scrape the pan so that all the juices go in as nut roasts can sometimes be a little dry.
Dissolve the yeast extract in hot water or stock and add to the other ingredients. Season well.
Melt the butter in a frying pan and saute the chopped mushrooms until soft.
Grease a 2 lb (900g) loaf tin with butter, then press in half the nut mixture.
Cover with a layer of mushrooms and top with the rest of the nut mixture.
Then cover with foil and bake for 1 hour.
When cooked, remove the loaf from the oven and let it stand for 10 minutes before turning it out.
Serve hot or cold.
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