Place flour and 1 teaspoon of salt in the bowl of a food processor fitted with the steel blade.
Cut butter into cubes and place in food processor. Pulse until butter is the size of peas.
Add the egg yolks and pulse a few times to combine.
With the motor running, add ice water through the feed tube and pulse until dough starts to form.
Remove from food processor and form the dough into a ball. Pat it semi-flat into a disk shape and cover with plastic wrap or put into ziptop bag. Chill for 30 minutes.
Preheat the oven to 400 degrees. Line a sheet pan with parchment paper.
While the dough is chilling, put the sliced tomatoes in a large bowl. Put the parsley, basil, garlic, thyme, salt and pepper (1 teaspoon each) in the bowl of a food processor and process until finely minced.
With the processor running, pour the olive oil down the feed tube and mix until combined.
Pour the basil mixture over the tomatoes. Gently toss the tomatoes until they are all covered witht he basil mixture. Set aside.
Take your chilled dough out of the refrigerator and roll the dough out to the size of your sheet pan (11 x 17 works well).
Grease the sheet pan with cooking spray and place the rolled out dough on the sheet pan.
Place a second sheet pan directly on the pastry and bake for 15 minutes.
Remove the top sheet pan and pierce the pastry in many places with a fork.
Bake again for 8 to 10 minutes, until lightly browned. Be sure to check the dough while it’s cooking and pierce any areas that start to bubble up.
Remove from the oven and let cool for 13-18 minutes.
Lower the oven temperature to 375 degrees.
Brush the mustard on the crust with a pastry brush or the back of a spoon.
Sprinkle a thick even layer of Gruyere on the dough, saving 1/2 cup of the cheese for the top.
Sprinkle 1/2 cup of Parmesan.
Place tomatoes on top. Overlap parts of the tomato in rows.
Pour any remaining herb mixture on top of the the tomatoes.
Sprinkle the remaining 1/2 cup of Gruyere and the some parmesan on top of the tomatoes.
Bake for 30 minutes final minutes.
Remove from oven and cool for 5 minutes before cutting into squares.