How to Make It
Warm flatbread according to package directions, allowing it to get slightly crisp around edges. Set aside.
Stir together softened cream cheese, capers, and herbs in a small bowl until combined. Spread both pieces of flatbread evenly with cream cheese mixture, leaving a 1/2-inch border.
Toss together arugula, olive oil, and l tablespoon lemon juice in a medium bowl until coated. Scatter dressed arugula over cream cheese mixture. Scatter drained tuna over arugula. Evenly scatter Parmesan and onions over tuna, sprinkle with pepper and lemon zest. Cut each flatbread into halves diagonally; serve immediately.