To make the cashew cream sauce, heat olive oil in a fry pan on low medium heat. Cook onion very gently, avoiding getting any colour on them, for about 2 minutes, until starting to soften. Transfer onion to a food processor or high speed blender, along with remaining sauce ingredients and blend until very smooth and creamy – this will take at least a few minutes. Scrape down the sides of the processor/blender with a spatula as needed. Add a bit more water or milk as required (probably about ¼ cup) to achieve a pouring cream consistency.
Bring a large pot of salted water to the boil for the pasta. Heat olive oil in a large fry pan on medium heat. Cook mushrooms, with a good pinch of salt, for about 3 minutes until starting to soften – if they are getting a bit dry, you can add a tablespoon or two of white wine or vegetable stock and they will soak it up, helping to keep them soft and moist. Add garlic and thyme to the mushrooms, and continue to cook for a further 1 minute. Set aside one third of the cooked mushroom mixture (this will be used for garnishing later).
Cook pasta according to packet instructions until al dente (just cooked thorugh). Add cashew cream sauce to the mushrooms in the pan and bring to a gentle simmer to heat through. Turn off the heat and stir through chives and parsley.
Scoop out about 1/3 cup of pasta cooking water and stir into the sauce. Drain pasta and divide between serving bowls. Spoon over cashew cream sauce and garnish with extra cooked mushrooms, chives, parsley and basil leaves (if using). Serve with rocket salad, dressed with avocado/olive oil and balsamic, on the side.