Grind the mustard seeds to a coarse powder, using a mortar and pestle. Cut each of the limes into eight pieces. Slit the chillies and cut them into 2 cm (¾ inch) pieces.
Toss the ground mustard seeds in a bowl with the turmeric, salt and chilli and lime pieces, mixing until well combined.
Transfer the mixture to a sterilised glass bottle or earthenware jar. Pour the vegetable oi