Place the cashews, water, nutritional yeast, mustard and garlic in the blender. Blend until smooth and creamy. Taste and add salt and pepper to taste.
Place a large frying pan over a medium heat. Add half the oil and fry the onion until golden. Remove the onions from the pan and return the pan to the heat. Increase the heat to medium high and add the mushrooms (you may need to cook them in batches) and the remaining half of the oil. Cook until lightly golden. Return the onions to the pan and add the thyme and the alfredo sauce.
While the mushrooms are cooking cook the pasta according to packet directions. When you add the sauce to the pan take a spoonful of the pasta water and stir through the sauce. Heat until warmed and then remove from the heat. Drain the pasta.
Toss the pasta through the sauce and serve immediately.
Nutritional yeast sounds (and even looks) a little scary but it adds a wonderful cheesy flavor to the pasta. Nutritional yeast is often called Savory Yeast Flakes. Note that brewers yeast is NOT the same thing so don't use it in its place.Could you use almonds milk in place of cashews? Yes, but I find that the almonds have a quite strong almond flavor whereas cashews tend to all but disappear amongst the other flavors in this sauce so I'd suggest sticking to cashews.For a gluten-free version, simply select a gluten-free pasta brand.