First make the sauce. Melt 1 oz (25 g) of the butter in a large pan and sweat the turbot bones, shallots and mushrooms for 5 minutes. Add the red wine, fish stock and veal stock, and reduce to one third of its original volume. Strain into a clean pan and reduce again until syrupy. Add the cream and bring to the boil. Whisk in the remaining butter, season and strain. Keep warm.
Pre-heat the oven to gas mark 5, 375°F (190°C). Mix together the spinach, mushrooms and butter for the pasty fillings. Divide the pastry into 4, and cut each piece out into a rectangle. Place a spoonful of the filling in the centre of each, brush the edges with egg and fold together to make a pasty. Glaze with the egg wash and bake in the oven for 10–15 minutes.
Fry the turbot fillets gently in oil. Pour a circle of sauce onto each serving centre with a spinach pasty to garnish.