Pre-heat the oven to gas mark 4 or 350F (180 °C). Heat the oil & fry the onion & garlic till soft. Grind the cashew nuts, then mix with the breadcrumbs. Mix in the mashed parsnips & herbs. Add the onion, being sure to scrape all the juices into the mixture. Dissolve the yeast in the water & mix into the vegetable & nut mixture. Season well. Melt the butter in a skillet & saute the chopped mushrooms until soft. Grease a 2 pound loaf tin then press in half the nut mixture, cover with a layer of mushrooms & top with the rest of the nut mixture. Press in well. Cover with foil & bake for 1 hour. When cooked, remove the pan & let stand for 10 minutes before turning onto a plate. Serve hot or cold. Slice to serve. Serve with vegetables or a crisp green salad.