Place the cashews in a heat-proof bowl. Cover with plenty of boiling water and set aside to soak for at least half an hour.
Place a large pan of salted water on to boil. Cook the pasta according to the packet directions then drain but reserve the cooking water.
While the pasta is cooking, pour the tablespoon of olive oil into a large pan over a low heat. Add the chopped red onion and cook gently, stirring every now and again, until the onion is soft and translucent, about 10 mins.
Add the sliced mushrooms, the crushed garlic and the thyme. Cook over a medium heat until the muhrooms are soft and the excess liquid has evaporated.
Add the white wine and continue to cook, stirring often, until it has evaporated.
Meanwhile, drain the soaked cashews and place them in a blender along with the 250ml water, the vegetable/mushroom stock cube and the nutritional yeast. Blend until very smooth.
Pour the cashew cream into the pan of mushrooms and stir in the truffle oil. Cook over a low heat for a couple of minutes, adding some of the reserved pasta cooking water as needed to form a silky, creamy sauce. I usually add about 2 ladlefuls of the pasta water.
Season to taste with plenty of salt and pepper then toss through the cooked pasta and serve topped with a sprinkle of chopped fresh parsley if you like.