How to Make It
Heat 1/3 cup of the olive oil in a large enameled cast-iron casserole. Add the onions, prosciutto, shallots, thyme, garlic and bay leaves and cook over moderately low heat, stirring, until the onions begin to soften, about 5 minutes. Reduce the heat to low and cook until softened, about 12 minutes.
Add the bell peppers and cook until softened, about 5 minutes. Add the eggplant and cook until softened, about 5 minutes. Add the zucchini and cook, stirring often, until softened, about 10 minutes. Add the wine and simmer over moderately high heat for 5 minutes. Add the tomatoes and simmer for 5 minutes longer, then add the V-8 juice and tomato paste. Simmer over moderate heat until thickened, about 15 minutes. Discard the thyme and bay leaves and season with salt and pepper.
Preheat the oven to 425°. Heat 2 large ovenproof skillets. Season the tuna steaks with salt and pepper. Add 2 tablespoons of the olive oil to each skillet. When the oil shimmers, add 2 tuna steaks to each skillet and cook over high heat until deeply browned on the bottom, about 4 minutes. Turn the steaks over and transfer the skillets to the oven. Roast for about 8 minutes, or until the tuna is pink inside. Transfer the tuna steaks to a carving board.
Ladle the sauce onto 2 large platters. Slice the tuna across the grain 1/2 -inch thick. Arrange the tuna on the sauce, pour the pan juices over the tuna and serve very warm but not steaming hot.