Place the quartered limes in a large bowl and sprinkle the salt over, cover with cling film and set aside for 2 days, stirring every now and then
Heat the oil in a medium-sized pot over a medium heat. Add the mustard seeds and sauté until the seeds start to pop. Stir in the garlic, ginger, cumin, coriander and chilli flakes. Stir for a further 5 minutes.
Add the limes, water, sugar and vinegar to the mix and bring to a boil. Lower the heat and allow to simmer, stirring now and then, for 15 minutes or until thick.
Spoon into sterilised glass jars, seal and set aside for 1 week to develop the flavours.