One of my very favourite things ever.
And really easy to make. Mix up a cup or so of sea salt with a few spices [fennel seeds, coriander, black pepper, bay leaves]. Clean the lemons and slice them lengthways into quarters, but not as far as the end so the four sections stay attached. Press on each end to open the lemons up. Do this over the salt mix bowl so the juice collects. Pack the salt mix in between the sections and put them into a clean jar with a tight seal. Pour the lemon juice from the bowl in too. You might add a little water too so they're covered. That's it. Leave them in the fridge for a month or two until the skins become soft. You can then use them to cook with fish, chicken, or in a tagine. They keep for a couple of years too.