To make the creamy cabbage, cook the cabbage in boiling salted water for 3 minutes and plunge into iced water. Drain and squeeze out the excess water. Pour the oil into a pan and add the carrot and celeriac, cook for 6 minutes, add the garlic, butter and bacon, cook for 2 minutes. Test that the carrot and celeriac are cooked, then add the cabbage, vinegar and warm through, season and add the sour cream, cook for 1 minute.
Season the mackerel pieces, pour the oil into a frying pan and cook the fish for 3 minutes until golden brown, flip over, add the butter and cook for a further 4mins basting often. Add a squeeze of lemon juice.
To serve, spoon the cabbage onto a plate and place a piece of cod on top.