Tear off four 12-inch long sheets of heavy-duty (long) foil. Press about half of 1 sheet into a shallow soup or pasta bowl to make a “bowl” out of the foil, leaving half of the sheet hanging over the side for folding over to make a pouch.
Drizzle in a couple of teaspoons of oil. Add a handful of leek slices and tomato halves, a pinch of tarragon and then a fish fillet. Pour in 2 tablespoons wine, add a 1-tablepoon pat of butter, another pinch of tarragon, and season well with salt and pepper. Fold the excess foil over top and crimp the edges together to enclose the fish and vegetables. Repeat with the remaining ingredients, saving about 1 tablespoon of tarragon for serving time. Place the pouches in a single layer on a large baking sheet. (You can prepare up to 4 hours ahead. Refrigerate until ready to cook.)
Preheat the oven or a gas grill to 400°F. Place the sheet with the pouches in the oven or on the grill and cover and cook for 25 minutes.
Unwrap one of the pouches, taking care to keep the juices from escaping. Using a spatula, transfer the fish to a shallow bowl or plate. Spoon the leek and tomato on top, pour on the juices from the foil, and sprinkle with some more fresh tarragon. Repeat with remaining fish portions.