Puff Pastry Tomato Tart

Prep the Tomatoes

1. Slice tomatoes crosswise into 1/4-inch thick rounds and place them on a paper towel-lined board or baking sheet. Lightly sprinkle the tomatoes with a few pinches of salt and top with another layer of paper towels.
2. Let stand 15 minutes (30, if you have the time), for the tomatoes to release their juices. Pat the tomatoes dry and proceed with preparing the tart.

Make the Tart

1. Preheat oven to 400 degrees F with the rack in the lower middle position. Line a baking sheet with parchment paper.
2. Unfold the defrosted puff pastry onto the prepared baking sheet. Lightly roll it to a 1/8-inch thick rectangle, measuring approximately 10x14 inches. (*See important note below.)
3. Use a knife to score a border around the puff pastry rectangle, about 3/4- to 1-inch from the edge. Be careful not to cut all the way through the pastry. Use a fork to prick the center area of the pastry all over ("docking"). Do not dock the scored border area.
4. Evenly spread the docked area of the pastry with Dijon mustard. Layer 1 cup of Gruyere, the caramelized onions, and half of the herbs over the mustard.
5. Place the tomatoes on the top of the tart in a single layer, slightly overlapping the sides of the slices, if needed. If desired, very lightly drizzle the tomatoes with olive oil (about 1 teaspoon worth).
6. Season the tart to taste with freshly-ground black pepper. Top the tart with the remaining 1/4 cup of Gruyere.
7. In a small bowl, whisk together the egg and water. Brush the border of the tart with the egg wash.***
8. Bake the tart for 25-30 minutes, until the pastry border is puffed and deeply golden brown, and the cheese is melted.
9. Remove the tart from the oven and let stand for 5 minutes. Sprinkle with the remaining herbs and season to taste with a pinch or two of sea salt (only if needed). Cut the tart into slices, and serve hot or warm.