Heat the oil in a small pan and fry the mustard seeds until they start to ‘pop'. Add the garlic and ginger and stir-fry for 1 minute before adding the remaining spices. Cook for further 2 minutes.
Stir in the limes, remove from the heat and add the vinegar. Mix well, then pour into a glass jar. When cool, seal the jar and place it somewhere warm for 4-6 days, shaking it daily. (Keep for 2 weeks in the fridge).
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