Cut the lemons lengthwise through the middle until nearly halved, but leaving 2 cm (1 in) or so by the stalk to make sure the lemon holds together. Make another incision at a 90° angle, so you have a cross cut (see pictures above if this is confusing).
Add plenty of coarse salt into the opening of the lemons, at least a couple of teaspoons per lemon. Pack them tightly in a sterilised jar.
Close and seal the jar and place in a window, preferably one that gets a bit of sunlight, for 2 days. By this point, the lemons should have released enough juices to cover everything. If they haven't, add more lemon juice until all the lemons are completely covered, then close and reseal the jar.
Leave the jar in the window for a total of 14 days, turning the jar occasionally (every day or two is fine).
After 14 days, transfer the jar to the fridge. You can use them already, but will be even better if you leave them for another 14 days before opening the jar.
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