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Slice the tomatoes (you can skin them if you want, but it's not necessary). Finely chop the onions. Layer both in a large bowl with the salt. Leave overnight.
The next day, chop the sultanas using a large, sharp knife, then peel, core and chop the apples. Put the sugar and vinegar into a large pan and bring to the boil, stirring to dissolve the sugar. Add the sultanas and apples and simmer for 10 mins. Strain the tomatoes and onions in a colander (but don’t rinse), then tip into the pan and return to the boil.
Simmer for about 1 hr, stirring occasionally until the mixture is thick and pulpy. Transfer to warmed jars (see tips, below) and cover with lids.