Peel tomatoes - Soak in boiling water for 2 minutes. Drain and cool a little. Skins should now be split and easily peel off. If not split, apply a little pressure or scratch at skin a little and it should peel off easily. If using cherry tomatoes, they should just pop out of their skin by applying a little pressure at the base of the tomato (see video).
Chop tomatoes and onion and place in a tray. Sprinkle with the salt and cover with a clean tea towel. Leave out (at room temperature) overnight.
The next day, drain off liquid and place tomatoes and onion in a saucepan along with the sugar and vinegar.
Heat over medium heat, stirring, to dissolve the sugar. Bring to the boil and boil for 10 minutes, stirring occasionally. Skim off any foam that forms on the top with a spoon.
Combine remaining ingredients in a small bowl to make a paste. Add to saucepan and stir well to combine.
Continue to simmer for another 20 minutes (approx), stirring from time to time to break up the tomatoes and make sure the mixture doesn't stick to the bottom of the saucepan and burn. The mixture should thicken to reach a jam-like consistency.
Pour into sterilised jars and allow to cool. Store in the fridge for up to 6 months.