Soak the cashew pieces in the refrigerator for a minimum of 8-12 hours, up to 24, then rinse well under cold running water.
Throw them in the bowl of your food processor, along with the rest of the ingredients and process until smooth and creamy, which should take about 5 to 8 minutes. You might have to stop and scrape the sides 2 or 3 times.
You can use this sour cream immediately or keep in the refrigerator in an airtight container, for up to a week.