Place the tomatoes and onions in a bowl. Sprinkle with the salt and leave overnight.
Once soaked, pour off the liquid. Place the tomatoes and onions into a large saucepan. Add the vinegar, sugar and chillies. Gently simmer for 30 minutes.
Combine the cornflour, curry and mustard powder with the water and mix to make a paste.
Gradually stir the paste into the cooking mixt