The following recipe is about as simple as recipes get - that is, in terms of ingredients: lemons and salt. BUT it is relatively time-consuming to get it right, and I don't recommend cutting corners.
You also need a large chopping board, a sharp knife (kitchen devil, serrated is what I recommend) and one of those large storage jars with a lever and a rubber seal.
Here are the steps. You need about six lemons to make this worthwhile but there is no reason for not adding more lemons (and salt) if you decide to make vast amounts of this. No need to double quantities; just a lemon, a couple of lemons...etc.
First, wash your lemons, then set one aside.
With each lemon (except for the one set aside), slice off the very ends, then chop it into quarters.
Remove all the pips. This is absolutely essential because if you don't, they will end up as bits of sliminess in your pickle.
Having removed the pips, get the lemons into even smaller pieces. Sixteenths or even thirty-seconds - but no need to be exact. Chop the lemon as though you were chopping an onion finely. Check for stray pips as you go along.
So you will end up with a pile of bits of lemon, peel and everything, on your chopping board.
When you finish chopping each lemon, tip it into your storage jar, scraping all the lemony stuff off the board into the jar, then add a layer of salt (table salt, not salt crystals).
Stir the whole mixture around and start work on the next lemon, repeating the above process.
The final step involves the lemon that you have NOT so far chopped.
Halve it and then juice it, ideally using one of those glass juicer things, because, again, the pips are definitely not wanted.
Pour the juice into the jar, then chop the squeezed lemon halves into similar sizes to the rest of the lemons already in the jar and add them.
Add some more salt and give it a stir.
Then put the lid on, label the jar with the date and put in the fridge.
Now it's a waiting game. Check the pickle about once a week. Stir it. May be worth adding more lemon juice if it's a little dry. Possibly even a little more salt.
Worth eating after about a month and it lasts indefinitely.
Mum once tried adding the remains of some Patak's lime pickle to it which chillified it but I was not convinced...
This is the only pickle I know that really can be eaten with just about anything: Shepherd's pie, casserole of any kind. "Can't go wrong", as my mother would say.