Place cashews in bowl, cover with water, and let soak for 3 to 8 hours. Drain and transfer to blender.
Add ¼ c rejuvelac and a pinch of salt, and blend until perfectly smooth. If it’s too dry, add a bit more rejuvelac, 1 t at a time.
Transfer to container, cover, and let culture at room temperature for 8 to 36 hours, tasting occasionally until it’s as sharp as you like it.