Cover potatoes and carrots with water in a saucepan. Bring to a gentle boil, then reduce heat to medium-low and simmer for 10-15 minutes, until potatoes are fork-tender. (If you haven't soaked your cashews yet, you can also boil them in the same saucepan at this time.)
Drain potatoes and carrots and add to a blender with remaining ingredients. Reserve the starchy cooking water to thin out your sauce if needed.
Blend until completely smooth, scraping down sides of blender with a spatula as needed. Depending on how starchy your potato is, you may need to add a little extra water to thin out the sauce. Add some of the reserved cooking water, a few tablespoons at a time, blending between additions until your perfect consistency is achieved.