Step 1

Shuck the kernels off the cobs (this is most easily done by standing each cob upright in a large bowl and running a knife down it, mowing the corn as you go). Cook the corn in plenty of boiling, salted water for about 2 minutes and drain well.

Step 2

In a saucepan, mix the onion, chillies, if using, coriander seeds, lime zest and juice, sugar, vinegar and salt. Bring to the boil and simmer hard for about 5 minutes. Reduce the heat, add the sweetcorn and simmer gently for 20 minutes.

Step 3

Mix the cornflour with 4 tablespoons water to make a paste. Add this to the sweetcorn mixture and bring the heat back up to a simmer for about 5 minutes or until the sauce has thickened. Transfer to a sterilised jar and cool. You can either eat it immediately or keep in a cool place for up to 3 months.