**Day one.** Layer the chopped tomatoes and onions in a large bowl, sprinkling the salt through as you go. Cover and leave overnight at room temperature.
**Day two.** Tip into a colander and drain thoroughly, pressing gently. Discard the liquid — there will be a lot. Don’t rinse.
Put the drained tomatoes and onions into a large heavy-based pot with the vinegar and sugar. Bring to the boil, stirring until the sugar dissolves.
Reduce to a steady simmer and cook, uncovered, for about 45 minutes, stirring occasionally, until visibly thickened and reduced.
Meanwhile, sterilise the jars: washed, rinsed, and dried in a 120°C oven for 15 minutes. Boil the lids separately for 5 minutes.
Mix the curry powder, mustard powder, ginger, cayenne, flour and cornflour with just enough cold water to make a smooth thin paste — about 1/2 cup. Whisk it into the relish.
Simmer a further 15–20 minutes, stirring more often now so it doesn’t catch, until a spoon dragged across the base leaves a channel that holds for a couple of seconds.
Ladle the hot relish into the hot jars, filling to within 1 cm of the top. Wipe the rims and seal immediately.
Cool completely, label with the date, and **leave at least a month before opening.**