Mix the zucchini, onions and chilli with the salt and leave to stand overnight, covered. Press into a sieve and drain well.
Mix with the remaining ingredients (except for the tomato puree and tomato paste), cook for 30 minutes and puree. Finally stir in the tomato puree and tomato paste and briefly bring to the boil again. Fill hot jars with screw-on lids and reduce if necessary.
Goes well with fondue, grilled meats and tortilla chips.