Rinse the cashews, place in blender with water and salt, and blend on high speed for 2 minutes.
Suspend the nut bag over a bowl (for example, you can set the bowl in the sink and hang the bag from the faucet) or set the colander over a bowl and line it with cheesecloth.
Pour the cashew milk into the bag / cheesecloth and gently knead or use a spoon to help it pass through. Set the solids aside to use for another recipe.
Stir the rejuvelac into the milk and transfer to the mason jar. Cover loosely and leave to culture at room temperature. Every few hours, tighten the lid and shake, then loosen the lid again to allow gas to escape.
After 24 hours, mix well and taste to decide whether it needs to culture for up to another 12 hours.