In a medium bowl combine mayonnaise, parsley, chives, tarragon, lemon peel, lemon juice, mustard, and pepper; stir until well combined. Stir in tuna, breaking up any large pieces with a fork.
Place 4 bread slices on work surface; evenly divide tuna mixture on bread slices. Top each with tomato, if desired, and cheese. Spread one side of remaining bread slices with half the butter. Place bread slices, buttered side up, atop cheese. Place sandwiches, buttered side down, on a large nonstick griddle over medium heat. (Or cook sandwiches, half at a time, in a large nonstick skillet.) Carefully butter top bread slices. Cook sandwiches for 6 to 8 minutes or until cheese is melted and bread is golden, carefully turning once halfway through cooking. Serve warm. Makes 4 sandwiches.