Put the turbot steaks in a pan with the onions and enough wine and stock to cover. Bring to a boil, then reduce the heat and poach gently for 15 minutes. Remove the fish to a serving dish and keep warm. Strain the liquid and reserve.
Mash the pickled walnuts. Combine them with the butter, 1 cup wine/stock mixture and the chopped watercress. Season. Bring to a boil and simmer for a few minutes. Pour this sauce over the fish, garnish with pickled walnut halves and serve.