Whisk together mayonnaise, parsley, chives, tarragon, lemon zest, lemon juice, Dijon mustard, and pepper in a bowl until well combined. Gently stir in the flaked tuna, cover, and refrigerate until chilled, about 30 minutes.
Spread tuna evenly over four of the bread slices. Top with tomatoes, and cheese if using, then top with remaining bread slices.
