Place the flour, salt and butter into a food processor and pulse until in small pea-size.

With the motor running, add the egg yolk and then add the water until the dough begins to combine. Place the dough onto a cutting board and roll into a ball, then flatten into a round disc shape. Wrap in plastic wrap and place in the refrigerator for 30 minutes.

Preheat the oven to 400 degrees Fahrenheit. Line a sheet pan with parchment paper in preparation for the crust.

While the dough is chilling, slice the tomatoes and place in a medium size bowl.

Make the dressing for the tomatoes: combine the parsley, basil, garlic, thyme, 1 teaspoon salt, and 1 teaspoon pepper in a food processor until minced. While the motor is running, pour the olive oil into the mixture until combined. Pour the mixture over the tomatoes.

Gently toss the tomatoes with the dressing and then set aside.

On a well-floured board, roll the dough out to an 10 x 10-inch square and transfer it to the prepared sheet pan.

Trim the rolled out dough so that it resembles a square, but don't be afraid to leave it looking rustic.

Place a second sheet pan on top of the rolled out dough and place in the oven for 15 minutes. Then, remove the second pan, poke the dough with a fork and return to the oven for 8-10 minutes. Remove from the oven when golden brown. Let cool for 15 minutes.

Reduce the oven temperature to 375 degrees Fahrenheit. Brush the Dijon mustard on the crust.

Leave about 1/2 - 1 inch of an edge without Dijon mustard.

Sprinkle half of the Gruyere on top of the Dijon mustard. Then sprinkle the Parmesan (reserve 2 tablespoons) on top of the Gruyere.

Place the tomatoes in overlapping rows on top of the cheese.

Spoon any extra dressing on top of the tomatoes. Sprinkle the remaining Gruyere and Parmesan on top of the tomatoes.

Bake for 30 minutes.

Cool slightly, cut into squares and serve warm or at room temperature. Serve with a small green salad and glass of crisp white wine or rosé.