1) Preheat the oven to 220C gas mark 7
2) Prick the potato all over and rub with a little of the oil and sprinkle with salt.
3) Put straight onto the shelf in the oven and cook for an hour or until the center is soft when prodded with a fork.
4) 10 minutes before the potato is due to come out of the oven, mix the red cabbage with the oil, season to taste and leave to one side.
5) Mix all of the other ingredients together except the butter and season.
6) Remove the potato from the oven and cut in half, top with the butter and then the mackerel mixture.
7) Serve with the red cabbage.