Place the all-purpose flour and ¼ teaspoon of kosher salt into the bowl of a food processor. Add the cold, cubed unsalted butter and pulse until the mixture resembles coarse, pea-sized crumbs.

Add the egg yolks and pulse to combine. Gradually stream in the ice-cold water, pulsing just until the dough begins to come together.

Turn the dough out onto a lightly floured marble surface. Pat it gently into a flat disk, wrap tightly in plastic wrap, and chill in the refrigerator for at least 30 minutes to relax the gluten and solidify the butter.

While the dough chills, arrange the ¼-inch heirloom tomato slices on a large platter or in a shallow bowl in a single layer.

In a clean food processor bowl, combine the fresh parsley, basil, dill, and garlic cloves. Pulse until finely chopped. Season with ½ teaspoon kosher salt and the coarse black pepper.

With the machine running, slowly pour in the olive oil to create a thick, bright green herb emulsion. Spread this mixture evenly over the resting tomato slices to marinate.

Preheat the oven to 400°F. Remove the chilled dough from the refrigerator and roll it out between two sheets of plastic wrap (or on a floured surface) to a ¼-inch thickness.

Transfer the dough to a baking sheet or a fluted tart pan, folding and crimping the edges to form a rustic crust.

Spray a sheet of aluminum foil with vegetable oil, place it oil-side down over the dough, and fill it heavily with pie weights or dried beans. Bake for 15 minutes.

Carefully remove the foil and weights, and bake until the crust is golden brown and structurally set, about 8 to 10 minutes more. (If the pastry bubbles, pierce gently with a fork). Remove and cool to room temperature.

Reduce the oven temperature to 375°F.

Brush the Dijon mustard evenly across the bottom of the cooled, blind-baked crust. (This acts as a critical protective barrier against the moisture of the tomatoes).

Scatter exactly half of the grated Gruyere cheese directly over the mustard layer.

Architecturally arrange the marinated heirloom tomato slices in a tight, overlapping pattern over the cheese. Top the tomatoes evenly with the remaining Gruyere and all of the grated Parmesan cheese.

Return the tart to the oven and bake until the cheese is completely melted, bubbling, and deeply golden, about 20 to 30 minutes. Allow to set briefly before slicing. Serve warm or at room temperature.