Strain the cashews and place them, along with the 3.5 cups of fresh water, in a high-speed blender. Blend for 90 seconds.
Add the guar gum and Prebio Plus to the blender for about 15-20 seconds until the mixture thickens. Check the temperature and let it cool to 100°F. Blending it can make the mixture hot.
Add this mixture to a bowl, preferably with a spout, and whisk in one sachet of L. Reuteri Superfood starter culture. This distributes the good bacteria throughout the mixture. Mix well for even distribution. Do not blend.
Place in jars, or jar, and cover lightly with plastic wrap or loose-fitting lids. Ferment at 98 - 100°F for 36 hours, away from the airflow of air vents/heaters/air conditioning, etc. Do not stir while fermenting.
When done, remove jars from the appliance, keep the lids loosely on the jars, and refrigerate. If you tighten your lids while the yogurt is still warm, it may build up pressure and cause your glass jars to break. Once the yogurt has chilled in the fridge, then you can tighten the lids.
This will keep in the fridge for up to 4 weeks. Separation is normal with cashew milk so just stir it back together before consuming it if you'd like.