Wash your apple, but avoid using soaps or produce wash. Using a cheese grater, grate the organic apple into semi-fine shreds. Use the skins, but discard the core.
Add 500 grams of brown rice or buckwheat flour, the grated apple, and 360 mL of lukewarm un-chlorinated water (about 1.5 cups) to a mixing bowl and thoroughly combine.
Transfer the mixture into a large glass airtight container that has enough room for it to at least double in size, minimum. (Ours usually quadruples while fermenting) Pack the mixture down into the bottom of the container. Close the lid.
To monitor growth, mark the side of your container with a washable marker or rubber band at the top level of the mixture.
Let the mixture sit for 3 days (72 hours) at a temperature of 70-75 degrees F. It should bubble and rise during this time.
After approximately 72 hours, thoroughly stir the mixture and then discard half of the amount. Then, thoroughly mix in another 250 grams of flour and 180 mL of tepid filtered water (about 3/4 cup) to the remaining starter mixture. This is called "feeding" the sourdough starter. You can do this either in a separate bowl, and put it back into a now-clean ferment vessel, or like we do, mix it in place.
Re-mark the container to note the height of the mixture. Let sit at 70-75 degrees, for 2 days or 48 hours this time.
After 48 hours, repeat the same discard and feed process as done previously. Discard half, feed, mix, mark the level, and cover again. This time, make it slightly more wet - using 250 grams of flour and about 220 mL of water.
Allow the sourdough starter mixture to sit for a final 24 hours.
Once complete, you now have a happy and active gluten-free sourdough starter! Store it in the refrigerator when not in use, taking it out to warm up and be fed weekly. OR if stored at room temperature, feed it daily to keep it alive.