Place the roughly 1-inch beet chunks in a clean wide-mouth glass fermentation vessel sized to the batch.
Add the mineral salt, then add filtered water while leaving about 2 inches of headspace. Disperse the salt without adding whey or another starter.
Set a clean glass, ceramic, or uncoated stainless fermentation weight over the beets so every solid remains below the liquid.
Fit an airlock or another self-venting, pressure-aware closure. Ferment at room temperature out of direct sunlight.
Begin checking around day 4 or 5. Continue until the liquid is deep ruby and tastes pleasantly tart. Use flavor as the endpoint rather than a taut lid, stopped gas, or routine burping.
Remove the weight, strain through a fine stainless-steel strainer securely nested over a glass bowl, transfer to a clean glass jar, cover, and refrigerate.