Wash lemons and cut about ½ cm off the top of each lemon. Cut each lemon from the top end into quarters part-way through, leaving them attached together at one end.
Transfer lemons to a large clean bowl. Using clean hands, open up lemons and rub insides with 3/4 of salt mix. Cover and set aside for 15 minutes to allow lemons to release some of their juice. Set aside remaining salt mix.
Transfer lemons to a sterilised jar, pushing down lemons as you go to release more of their juice. Sprinkle with any remaining salt mix as you add each layer. Pour over lemon juice from the bowl (to submerge the lemons in juice as much as possible).
Seal jar and leave to preserve at cool room temperature for 2-3 weeks, until lemon rinds have softened and is ready to use. Store preserved lemons in the fridge once they have been opened. For use in cooking, scrape away the fleshy inside and slice or mince the lemon rind.