Melt vegan butter in a large skillet over medium-high heat. Optionally, add red pepper flakes and allow to bloom for about 30 seconds.
Add diced onions/shallot (optional) and minced garlic and sauté for 3-5 minutes, until onion is translucent.
Add cashew cream, vegan parmesan, and 1/2 cup reserved pasta water and stir to melt cheese. If the sauce begins to spatter too much, you can reduce the heat to medium or medium-low at this stage.
Add pasta and toss to coat, adding additional pasta water as needed to thin out the sauce to your preferred consistency. (If you anticipate having a lot of leftovers, see the notes about storage and reheating. You may wish to keep the sauce and pasta separate.)
Season liberally with salt to taste and enjoy!