Remove the outer leaves of the cabbage and set them aside. Cut the cabbage in half from head to core, then finely shred or chop it along with the fennel bulbs, onion, and apples (this works best in a food processor). Transfer everything to a large mixing bowl, add the salt, and gently massage until softened. Let it sit for 30–60 minutes until it starts to release moisture, then give it one final mix.
Add the sultanas and spices, mixing them thoroughly into the fennel mixture.
Begin filling your jar by taking a handful of the mixture at a time and pressing it down firmly with your fist. With each handful, you’ll notice more liquid being released. Fill the jar until it's about 2.5 cm from the top. To ensure successful fermentation, it’s essential to keep the cabbage submerged, so place a weight on top.
Allow it to ferment for 1–6 weeks, tasting it every couple of days to monitor the progress. If you're using an airtight jar, remember to ‘burp’ it every 2-3 days to release the build-up of carbon dioxide.
Once you're happy with the flavour and texture, store the jar in the fridge.