Score a cross in 8-10 unwaxed lemons (or use soapy water to wash off the wax), cutting right through almost to the base so they open up like a flower. Fill each with a very large pinch or two of flaked sea salt (this seems excessive but it’s fine) and close up.
Place a 1 litre glass kilner jar in the oven at 140c for 20 minutes to kill off any microbes then stuff the lemons into the jar firmly, leaving as little space for air as possible (I do this wearing lab gloves to reduce possible contamination).
After a week or so (on a windowsill) you will need to push the lemons down as the juice is released. This will mean they sit below the juice line and won’t dry out or get contaminated. When they sit comfortably below, pour an inch of oil over the top to form an airtight barrier and store in a cool place.
Ready to use after a month.
Traditionally, very small lemons were used for this job. In the UK our lemons are large, so if the recipe calls for a small one, I use just a quarter. But for a 4 person recipe, 1 whole lemon usually works. Remember to scrape off and discard the inner tissue and only use the rind when cooking.