Wash zucchini, trim ends and slice very thinly. Peel and quarter the onions lengthways and separate into layers. Combine zucchini, onions and salt with enough water to cover in a large bowl; stir well. Stand for 1 hour then drain.

Combine remaining ingredients in large saucepan or boiler. Bring to the boil, stirring until sugar has dissolved, pour hot vinegar mixture over zucchini mixture and stand 1 hour.

Transfer zucchini and vinegar mixture to the large saucepan. Bring to the boil and simmer for 3 minutes.

Spoon into warm, sterilised jars ensuring zucchini is covered with cooking liquid. Seal. When cold, label and store in a cool, dry place. Refrigerate after opening.