Blanch the tomatoes in boiling water then plunge into cold water to remove skins. Chop tomatoes and onions and then place into a bowl with the salt. Cover, and leave for 6-8 hours. Discard the water that accumulates in the bowl.
Place the onions, tomatoes, sugar, mustard, curry, cayenne and vinegar in a saucepan and bring to boil. Reduce heat, and simmer for 60-90 minutes until mixture has thickened and reduced by at least half.
Dilute the flour with a small amount of water to make a paste. Stir out any lumps, then quickly whisk this into the relish to thicken it.
Pour relish into a clean glass jar and store for up to two weeks in the refrigerator.