Whole limes boiled in brine and sun-dried until hard and hollow — limoo amani in Iran, loomi in the Gulf. Sour, musky and fermented, the sourness of ghormeh sabzi and Gulf machboos.
Persian and Middle Eastern grocers, whole or ground.
Worth knowing
Pierce them a few times so the stew gets in, and press them against the pot towards the end to release their juice. If grinding, crack them and pick out the seeds — they're bitter.