Prep all your vegetables so they are ready and on hand, this will make life a little easier. When cutting my red cabbage I found it best to quarter it then cut out the central core and slice what was left.
Melt the butter in a large pan over a medium heat, add in the finely chopped onion and fry for 4 to 5 minutes. Keep an eye on them so they don't start to fry but instead go soft.
Add the dark brown sugar, red wine vinegar, mixed spice, salt & pepper and water to the pan. Give it all a quick stir to mix it all together.
Add in the cabbage, stir then cover for pot and turn down the heat to a medium low level.
Return to the pot after 10 minutes to give it another stir.
After 20 minutes check to see how the cabbage is doing, give it another stir and check to see if it is almost at that perfect tender consistency. Still a little bit stiff in the middle but softening around the edges. If it's still rock hard give it another 10 minutes but not much more.
When you are happy with it add in the blackberries reduce the heat to low cover the pot again and leave for a further 10 minutes.
If eating immediately either serve from the pan or decant it into a serving dish.
Can be made up to a day in advance. Once cooked keep in the fridge and eat by the 3rd day reheating either using a microwave or in a pan over a gentle heat.