Cut the limes in half from top to bottom but don't cut all the way through, then turn the lime 45 degrees and cut from top to bottom again, again making sure you don't cut all the way through. You want your slices to remain joined at one end. Pack them with the salt and sugar in mason jars, squashing well. Seal the lids, shake, and leave to ferment for 1 month.
After that time, heat the oil and spices in a pan over low heat for a few minutes. Add the fermented limes along with all their juice and cook for 5 minutes. Allow to cool and then pack into fresh mason jars. Store for 1 week before using, and after opening keep in the fridge for up to 2 months.