Cut each lime into eights. Add to a large bowl. Massage the salt into the lime. Cover and leave at room temperature to ferment overnight.
Set a frying pan over a medium heat. Add the cumin, fennel, coriander, mustard and fenugreek seeds. Lightly toast until just fragrant. Remove from the heat and grind to a powder in a coffee grinder or a pestle and mortar.
Gently heat the oil in a frying pan until just hot. Remove from the heat. Add all the ground spices as well as the garam masala, chilli powder and nigella seeds
Stir in the jaggery. Cool the spiced oil thoroughly and add the fermented lime and the salty brine. Mix well and pack into sterilized jars.
Shake the jar(s) well and keep in the sun for a week. Leave to mature for another week before eating. The pickle will keep for up to 6 months at room temperature. Once opened, store in the fridge and consume within 2 weeks.