Melt half the butter in a shallow flameproof casserole dish that is big enough to hold all the pieces of cod in a single layer.
Fry the cod, skin-side down, for 1 minute, until crisp and golden. Carefully remove from the dish and set aside.
Add the onion to the dish and fry gently for 5 minutes, until softened.
Add the plum tomatoes, sun-dried tomatoes, stock and white wine. Bring to the boil and simmer for 10 minutes, until slightly reduced and thickened.
Preheat the oven to 200C/400F/Gas 6.
Place the cod skin-side up on top of the onion and tomato mixture, transfer the casserole dish to the oven and bake for 5-6 minutes - less if the pieces of cod are thin.
Remove from the oven and lift the cod out on to a warm plate. Return the dish to the stove over a high heat, add the remaining butter and reduce by boiling rapidly for about 4 minutes, stirring now and then, to make a really thick sauce.
Stir in the parsley, season with salt and pepper and cook for 30 seconds more. Spoon the sauce on to four warm plates, put the cod on top and garnish with parsley sprigs. Serve with boiled potatoes and green salad.