Preheat the oven to 180C/350F/Gas 4. Separate the cabbage heart by removing the looser leaves from the outer layers but leaving the tighter centre leaves attached to the stem. Bring a large pan of water to the boil, add the outer cabbage leaves and cook until tender. Remove with a slotted spoon into a bowl of iced water to refresh. Return the water to the boil and cook the centre leaves until tender then drain and refresh in iced water.
Line a baking tray with a large piece of buttered greaseproof paper, season the turbot and place on the paper. Fold the paper over like a parcel and carefully turn it over so the join is at the bottom. Place in the oven and cook for 7-8 minutes until the fish is just cooked through.
Put the large blanched cabbage leaves, pine nuts, parmesan and oil in a blender, pulse until smooth then season with sea salt and freshly ground black pepper and lemon juice.
Meanwhile, cook the potatoes in a pan of boiling water then drain well and peel. In a small pan, melt the Poacher cheese with the crème frâiche. Coat the potatoes in the melted cheese and check the seasoning. Cut the blanched cabbage centre into quarters. Melt a little butter in a pan and fry the cabbage until it starts to colour.
To serve, place the cabbage on a plate and top with a portion of turbot. Arrange the potatoes around the fish, spoon over the cabbage pesto and garnish with the chickweed.