Place beets, whey, and salt in a 2-quart glass container. Add filtered water to fill the container. Stir well and cover securely. Keep at room temperature for 2 days before transferring to refrigerator.
When most of the liquid has been drunk, you may fill up the container with water and keep at room temperature another 2 days. The resulting brew will be slightly less strong than the first. After the second brew, discard the beets and start again. You may, however reserve some of the liquid and use this as your inoculant instead of the whey.
Note: do not use grated beets in the preparation of beet tonic. When grated, beets exude too much juice resulting in a too rapid fermentation that favors the production of alcohol rather than lactic acid.
Recipe by Kelly the Kitchen Kop at https://kellythekitchenkop.com/so-good-about-beets-recipes-for-beets/