Before you start cooking – in a heat proof bowl, cover raw cashews with just boiled water for at least 30 minutes, or up to an hour.
Bring a large pot of salted water to boil. Cook pasta until underdone by about 3 minutes – it will continue to cook with the sauce later in the recipe. Reserve 1 cup of pasta cooking water, drain, and set aside.
Heat olive oil in a large skillet over medium heat. Add mushrooms, onions, and garlic, and cook for 8-10 minutes, until mushrooms are tender and onions are soft and caramelized.