1 Drain the tuna, taking care to collect the oil. Measure 2 tablespoons oil and mash finely with the tuna in a bowl. Season with pepper.
2 Rinse the tomatoes and cut into quarters, removing the stem ends. Cut the tomato quarters into slices.
3 Rinse, trim and coarsely grate the zucchini.
4 Cut flatbread in half with a bread knife and spread both halves with the tuna mixture.
5 Spread half of the tomato and zucchini on the bottom half of bread. Lightly season with salt and pepper and top with cheese slices.
6 Add remaining tomatoes and zucchini. Place the other piece of flatbread on top and wrap in aluminum foil.
7 Line a baking sheet with parchment paper. Place the bread in a preheated oven at 180°C (fan 160°C, gas mark 2-3) (approximately 350°F/convection 325°F) and bake on the middle rack for 20 minutes.
8 Remove aluminum foil and put back on the sheet. Bake until crispy, 5-7 minutes. Cut with a bread knife into serving pieces and serve while still warm.