Wash and dry the lemons. Cut a deep cross into the lemons from the blossom end to create quarters, but do not go all the way through the fruit. Push the quarters apart a little and cover them with salt. Layer the lemons as tightly as possible in the preserving jars.
Bring everything to the boil together, pour the liquid over the lemons (add some coriander, fennel and/or cumin seeds, and cloves and black peppercorns if you wish). The lemons must be fully covered with the liquid. Seal the jars, store in a cool, dark place. The lemons need to marinade in the liquid for at least 4 weeks to ensure they have the right flavour.