Preheat the oven to 425°F. Position a rack in the center of the oven. Place a pizza stone or baking sheet turnedupside down on the rack while the oven preheats.
Place the pastry round on a sheet of parchment paper on the counter. Arrange the tomato slices in two circles starting from the middle of the pastry, slightly overlapping the tomatoes. Be sure to leave a minimum 1®ˆ-inch border around the outeredge of the dough. Sprinkle the tomatoes with ¼ teaspoon of salt and ⅛ teaspoon of black pepper.
In a small bowl, combine the mozzarella, mayonnaise, herbs, remaining ¼ teaspoon of salt, and remaining ⅛ teaspoon of black pepper. Place dollops of the cheese mixture over the tomatoes.
Fold the excess dough over the tomatoes to form a crust border. Optional: baste the top of the crust with the egg white and lightly sprinkle with the poppy seeds and sesame seeds. Transfer the galette on the parchment paper to the preheated pizza stone or baking sheet. Bake for 20 to 25 minutes, or until the crust is golden and the cheese is melted and bubbling.
Remove from the oven and let cool for 15 minutes before cutting.