Combine mayonniase, parsley, scallions, tarragon, lemon peel, lemon juice, mustard, and pepper. Stir till combined well. Now stir in tuna, breaking up any large pieces.
Place 4 bread slices on a clean work surface, and evenly divide the tuna mixture on top of each slice. Top the tuna with tomato, if using, and cheese. Spread one side of the remaining bread slices with half the butter. Place bread slices, buttered side up, on top of cheese. Place sandwiches, buttered side down, on a large nonstick skillet or griddle, over medium heat. Carefully butter top bread slices. Cook sandwiches for 6 to 8 minutes or until cheese is melted and bread is golden, carefully turning halfway through cooking. Serve warm. Enjoy!