Prep: Wash your hands well, and then wash all of the lemons and set aside in a clean colander or bowl. Using a sharp paring knife or chef's knife, slice the ends off all but one lemon, saving it aside for juicing later on if necessary. Cut each lemon lengthwise as if you're going to quarter it, but stop just short of cutting all the way though.
Salt: Using your hands or a clean spoon, sprinkle a generous amount of salt over each sliced lemon. You can pry apart the quartered sections a little first to really get the salt in and around the insides of each lemon. Next, sprinkle a good coating of salt into a tall quart-sized Mason jar. Then, place each well-salted lemon into the jar, sprinkling with some more salt as you go along.
Brine: Finally, use a clean potato masher to press down hard on the lemons in order to squeeze out as much of their juices as possible. (I used the masher from my Vitamix to press down on the lemons.) Keep mashing until you have enough juice to cover the tops of the lemons completely. If necessary, juice the extra lemon you set aside earlier into the jar to top off the liquid.
Ferment: Once you've covered all of the skins with enough juice, lightly cover the jar with a lid and set it aside at room temperature for 2-4 weeks. Then, move your covered Mason jar to the fridge to stop the fermentation process.
Enjoy: To use your lemons in cooking, remove one from the jar, rinse well and remove any seeds, and finely chop the rind to add to your favorite recipes. Preserved lemons will stay good in a covered jar in the fridge for up to 12 months, or you can store the rinds in the freezer for up to 2 years.
Notes
Extra lemon: Save one lemon aside while you prep the rest; you may need a little extra fresh lemon juice to top off your ferment.Mason jars: Wide-mouthed Mason jars are easiest for fermenting, since they allow more room to get your hand in and out of the jar as you prepare your ferment.Softening the lemons: You can let the salted lemons sit a little in the jar first before you start mashing (and/or partway through) so that they naturally start to soften and break down from the salt.Fermentation time: If your kitchen is on the warmer side (or in the summertime), ferment for closer to 2 weeks, and if it's cooler ferment for up to 4 weeks.