Chop beets and apple into roughly 1-inch chunks. Avoid grating — too much surface area can lead to overly fast (alcoholic) fermentation.
Place cinnamon stick,orange peel, and ginger into a clean 1-quart glass jar. Add in the chopped beets and apples. Leave about 1 inch headspace at the top.
Dissolve 1 tablespoon sea salt in a little warm filtered water, then top up with cool water to fill the jar. Pour over the beets and apple until everything is submerged. Give it a gentle stir.
Screw the lid on and leave at room temperature for 2-3 days.
Once a day, stir the kvass with a spoon. Alternatively, use a fermentation weight so everything stays under the brine. After about a day you should see some bubbles rise to the surface.
The kvass is ready when it tastes sweet and pleasantly tangy, with a note of spice. This usually takes 2-3 days if your kitchen is warm.
Once ready, transfer the kvass to the refrigerator. If stored properly, it will keep for 2-3 months. The beets and apple can be reused once for a second brew — just add fresh saltwater brine and ferment 2–3 days.