Lime chutney (satini sitron vèr)

Instructions

1. Wash the limes, cut the zest into fine strips, then squeeze them to collect the juice.
2. Slice the onion and green chillies very finely, removing the seeds if you want it milder.
3. Dry-toast the mustard seeds in a small pan until they crackle, about 1 minute.
4. In a bowl, mix the lime juice, zest, onion, chilli, sugar and salt until dissolved.
5. Add the mustard seeds and cumin, then let it macerate for 30 minutes at room temperature, stirring now and then.
6. Transfer to a clean jar and keep in the fridge for up to a week.