Score a shallow cross into the skin at the bottom of each tomato. Fill one large bowl with boiling water and another with iced water. Place the tomatoes into the boiling water and allow to sit for 30 seconds then drain and transfer to the ice water for 30 seconds, gently remove skins.
Dry and place into a bowl with the onions, sprinkle over salt then cover and stand overnight.
Drain the liquid from the tomato mixture and transfer to a saucepan with 1 cup of the vinegar. Heat on medium and bring to a boil then simmer for 30 minutes.
Add the sugar and chilli, then simmer a further 1 hour, stirring occasionally.
In a bowl, combine remaining vinegar with mustard seeds, sumac and curry powder. stir into the tomato mixture and use a spoon to gently crush the tomatoes. remove from heat and allow to cool. Pour into sterilised jars to serve.