1. Preheat oven to hot (400° F.).
2. Sprinkle the fish inside with salt and pepper.
3. In a skillet heat one-third cup of the butter, add the onion and brown lightly. Add the crumbs, cucumber, capers, sage and half the wine and mix. Stuff the fish with the mixture and close with skewers and string.
4. Place the fish in a well-greased shallow pan lined with foil Cut gashes at one-serving intervals and place a piece of salt pork or bacon in each. Brush with the remaining butter and pour the remaining wine over the top.
5. Bake, uncovered, until the fish flakes easily when tested with a fork, about fifty minutes. Baste frequently with the drippings in the pan or with additional butter and wine.