Peel onions by first cutting an X in the blossom end, scalding briefly in boiling water then plunging into ice water. Peel, and cut into cubes then place in a bowl.
Place the chopped onions in another bowl, then sprinkle them and the tomatoes generously with salt. Cover each bowl with a clean tea towel and leave overnight.
The next day, place tomatoes, onions and their juices in a large saucepan. Add sugar, stir until dissolved over low heat, then increase heat and boil for 5 minutes, stirring frequently.
In a small bowl, combine curry, chilli, mustard and cornstarch, mixing in a little vinegar to make a smooth paste. Add it and the remaining vinegar to the saucepan and stir to combine thoroughly. Stir in the garlic, parsley, basil and oregano and bring to boil. Reduce heat until gently boiling, and cook uncovered for 50 to 60 minutes, or until thickened.
Pour into hot sterilized jars leaving 1/2 headspace, seal and process in a hot water bath or pressure canner. Store in a cool, dark cupboard for up to one year.