Peel and chop the apples. Peel and slice the onions. Drop 3 or 4 tamarillos at a time into a saucepan of boiling water. Leave for 20 seconds, lift them out with a slotted spoon and transfer to a bowl of cold water. Remove the peel and stalk, then chop the flesh coarsely. Put the fruit in a large saucepan with the apples and onions.
Mix in the remaining ingredients, then bring slowly to the boil, stirring occasionally. Simmer, uncovered, for at least 1 1/2 hours (but it may take up to 3 hours), or until thick and syrupy, and a deep browny-red in colour (it should reduce in volume by about a quarter). Towards the end of cooking, stir the chutney often to prevent it catching on the bottom of the pan. Ladle into clean, warm, dry jars. Seal with lids and leave until cool. When cold, label and date the jars. This chutney keeps well, but must be refrigerated.