Bring the water to the boil in a large saucepan. Add the whole limes and turmeric. Simmer for 2-3 minutes — this softens the skin and reduces bitterness. Remove from heat and allow to cool completely.
Once cooled, cut each lime into wedges — quarters or sixths depending on size. Place in a large bowl or jar, add the salt and mix thoroughly. Cover and leave for 24 hours. The salt draws out moisture from the limes and begins the pickling process.
The following day, heat 1 tablespoon of the oil in a pan over medium heat. Add the mustard seeds and wait for them to begin popping — this takes about 30 seconds. Add the curry leaves (they will spit) and fry for another 30 seconds.
Add the sliced green chillies and garlic. Fry for 1 minute until softened but not browned.
Add the salted lime wedges and red chilli powder. Stir to coat everything and fry on a high heat for 2 minutes, stirring constantly.
Remove from heat and allow to cool completely.
Pack the lime mixture into sterilised jars. Pour the remaining oil over the top to cover — this acts as a preservative and seal.
Seal tightly. The pickle can be eaten immediately but improves significantly after a few days as the flavours develop.
