4 medium-size ripe tomatoes
2 tablespoons olive oil
2 pounds fresh tuna steaks, 1 to 1½ inches thick, divided into 4 equal portions
½ cup finely chopped onions
3 tablespoons balsamic vinegar
3 tablespoons dry red wine
1 tablespoon minced fresh rosemary
Salt and freshly ground black pepper
¼ cup (1/2 stick) unsalted margarine in small pieces (optional)
Seed and finely chop the tomatoes. Place in a colander over a bowl and allow the excess liquid to drain at least 30 minutes. Set aside.
Brush a heavy skillet, large enough to hold the fish without crowding, with a little of the olive oil and place over high heat. Heat oil until very hot. Dry the tuna pieces and sear them until they are browned on the outside and still pink in the middle, about 3 minutes on each side. Transfer the fish to a platter, cover loosely with foil and keep warm.
Add the remaining olive oil to the skillet. Sauté the onions over low heat until they are tender but not browned. Add the tomatoes and cook until softened, about 5 minutes. Add the vinegar, wine and rosemary and cook over medium-high heat until most of the liquid in the pan has evaporated. Season to taste with salt and pepper.
If desired just before serving, over very low hear with the mixture barely simmering, stir in the margarine bit by bit. The sauce should become slightly creamy. When all the margarine is incorporated, return the fish to the pan, baste with the sauce, then serve immediately. Once the margarine has been added to the pan it is important not to allow the sauce to boil.