Halve the apricots and discard their stones. Put them into a large, deep, heavy-based saucepan. Cut the tomatoes in half and add them to the pot, then peel and roughly chop the onion and stir into the apricots.
Add the vinegar, sugar, sultanas, lemon zest and salt and bring to the boil. Stir the mixture well, lower the heat to a simmer, partially cover with a lid, then leave to bubble gently for an hour or so.
Stir in the harissa and honey and continue cooking for 10 minutes, then taste and correct the seasoning, if necessary, with salt, honey or lemon juice as you wish. Ladle into sterilised jars, seal and store in a cool, dark place.