1. Preheat oven to 400 degrees. Line a large baking sheet with parchment paper.
2. Carefully unfold the defrosted pastry sheet and place it on the parchment paper.
3. Use your hands to gently stretch the pastry to an approximate 16 inch square.
4. Score a line ½ inch in from all sides with a knife to make the tart border, do not cut all the way through the pastry.
5. Prick the sheet of pastry well with a fork, excluding the border. This prevents the pastry from puffing up as it’s baked.
6. Spread a thin layer of Dijon mustard on the inside of the tart, avoiding the border.
7. Brush the border with the egg wash.
8. Bake for 15 minutes or until golden brown. If pastry begins to puff in the center, gently press it down.
9. While baking, make the tomato herb topping.
10. Gently toss tomatoes in a colander to release frothy liquid and drain the excess juices and set aside.
11. In a medium bowl, add the olive oil, capers, chopped fresh herbs, minced garlic, ground fennel, dried red pepper flakes, salt, black pepper, and granulated sugar. Whisk ingredients well, then stir in the tomatoes that have been drained of their juices.
12. Assemble the pre-baked tart. Inside of the tart border, sprinkle Parmigiano Reggiano and Gruyere cheeses over the bottom of the tart.
13. Place the marinated tomatoes in a single layer, cut side facing up. Pour any remaining oil on top of the tomatoes.
14. Sprinkle mozzarella cheese on top of the tomatoes.
15. Return the tart to the 400 degree oven, baking it until crust is golden brown, approximately 15 minutes.
16. Remove from oven, let cool slightly, then garnish with additional herbs, cut, and serve.
