Clean and sanitize your canning jar in the dishwasher. Rinse and dry your whole lemons.
Cut off the blossom and stem ends off two lemons. Cut the lemons into slices that are ¼-inch (0.64 cm) thick. Save the lemon ends for another recipe. (Infused water or candied lemon peels are two possibilities.)
Juice enough of the remaining lemons to yield ¾ cup of fresh lemon juice.
Rub your lemon slices on all sides with salt. Don’t cut back on the amount of salt in this recipe. It helps prevent bad bacteria growth during the fermentation process.
Pack the salted lemon slices tightly into a jar, but don’t overfill. The slices must remain below the lip of the jar. Use fewer lemons than you sliced, if needed.
Cover the sliced lemons in the jar completely with lemon juice. You may or may not need all of the lemon juice you squeezed.
Place the jar weight (from the fermentation kit) in the jar to help keep the lemons under the liquid. Screw on the air lock lid. Add water to the lid if necessary, if the lid you have requires it.
Allow your lemons to ferment for 4-5 weeks at room temperature (about 60-70°F/16-21°C). Compare the pics of fresh to finished lemons in this article for visual indicators of "doneness."
If you are using an air lock lid and jar weight, you should not need to do anything with the lemons while they ferment. The one exception is if you are using an air lock lid with water, and your house is dry. In that case, you may need to add water to the lid periodically.
Refrigerate the fermented lemons after the fermentation period has ended. (Use a regular canning jar lid for storage, not the airlock lid.) They will keep in the fridge for 6-12 months, as long as they remain submerged in the brine. Enjoy!