Wash your beets with cool water. Quarter and slice them 1/4 of an inch thick.
To make the brine, mix and stir the filtered water and salt in a bowl or measuring cup, until the salt is fully dissolved.
Pack the sliced beets, garlic, and rosemary sprigs into the quart jar. Pour the brine over the beets, leaving some headspace.
Add a fermentation weight to keep the beets below the brine. Cover loosely with a lid.
Leave on the countertop at room temperature for 4-7 days.
When the beets are finished fermenting, remove the fermenting weight, cover with a lid, and store in the refrigerator for up to three months.
Bon Appétite!