Dice the tomatoes and onions finely and cover with water.
Add salt and leave overnight.
Drain the tomato mixture and place in the slow cooker
In a bowl combine sugar, curry paste, mustard powder and turmeric and mix to a paste.
Stir it into the tomato mixture in the slow cooker. Pour in enough white vinegar to cover the mixture.
Cook on low for approx 6 hours
When almost cooked mix plain flour with samll amount of water to a paste and stir in to thicken.
Allow to cool and pour into clean sterilised jars.