Preheat the oven to 425F. Lay the prosciutto on a flat surface with the slices slightly overlapping. Put the fish on top and wrap it in the prosciutto with the ends overlapping. Turn it over and set aside.

Heat the oil in a large skillet. Add the mushrooms and a pinch of salt and cook until browned, 3-5 minutes. Stir in the garlic, then add the wine, and cook over high heat for 1 minute. Stir in the tomatoes, salt lightly and simmer gently for 10 or 15 minutes until thickened.

Pour the sauce into a baking dish large enough to hold the fish. Set the fish on top and roast for 15 minutes. Lower the temperature to 400F and roast for 30 minutes more. Remove from the oven and put the fish on a plate. Stir the sour cream and basil into the tomatoes. Set the fish on top and serve.