Add all of the ingredients to a high-powered blender, like a Vitamix, and blend until completely smooth. If you don't own a powerful blender, you should soak the cashews for several hours or boil them first. Drain them and then proceed.
After blending, scrape down the sides and blend again. You want it 100% smooth like a heavy cream. It should be super creamy and beautifully tart.
You can use immediately or chill in the fridge a few hours first. It will thicken up a lot more in the fridge similar to a sour cream texture. The sauce will last about 4 days in the fridge.
See post above for many uses for this amazing vegan lemon cream sauce.