For the poached turbot, place the turbot fillet into a deep frying pan, pour over enough water to cover and bring to the boil. Reduce the heat and simmer gently for 4-5 minutes, or until the turbot is just cooked. Remove from the water with a slotted spoon and place onto a warmed serving plate.
Meanwhile, for the sauce, heat the butter in a small saucepan, add the white wine and simmer until the liquid has reduced by half.
Add the fish stock and simmer until the liquid has reduced by half.
Add the double cream and cook gently for 1-2 minutes. Season, to taste, with salt and freshly ground black pepper and stir well.
To serve, pour the white wine sauce over the turbot.