Soak the cashews: boil a small pot of water, place the cashews in a medium sized bowl, and pour the boiling water over. Let sit for at least 30 minutes, an hour preferred.
Heat your oven to 425 degrees. Place the peeled and chopped butternut squash on a baking pan, and pour the olive oil on top. Coat well, and then roast for 20 minutes.
Turn the butternut squash in the pan in order to prevent burning, and add the chopped carrots. Roast for an additional 20 minutes.
Fill a large soup pot with water and bring to a boil. Add a dash of salt (or more, to your liking), and cook the pasta according to package directions.
Rinse and drain the cashews.
In a large blender or Vitamix, add the onion, garlic, carrots, butternut squash, and 1 c. of the pasta water. Blend on high until smooth.
Drain the pasta, and then place back in the soup pot. Mix the butternut sauce with the pasta, and add in any additional ingredients you prefer. (We enjoyed this on it's own, with spinach added one night, and with sage added another night). Serve immediately.