Peel off the outer leaves of the cabbage, then cut into quarters and slice out the core. Use a sharp knife or the slicing attachment of a food processor to thinly slice the cabbage.
Heat the butter in a large saucepan, then tip in the onions and gently fry until softened, about 5 mins. Add the orange zest to the pan along with the cinnamon stick, then cook for 1 min more. Add the shredded cabbage, then pour over the port, red wine vinegar, orange juice and 150ml water. Bring up to the boil, then reduce the heat to a simmer, cover the pan and cook for 45 mins-1 hr until the cabbage is softened.