Step 1

Preheat the oven to 400 degrees F (200 degrees C). Line a sheet pan with parchment paper.

Step 2

Allow the dough to come to room temperature, about 20 minutes.

Step 3

Meanwhile, place sliced tomatoes in a single layer on a paper towel-lined plate, and place zucchini and yellow squash slices on plates as well, also in a single layer. Sprinkle vegetables lightly with salt, about 1/4 teaspoon for each vegetable. Allow to stand for 20 minutes.

Step 4

Melt butter in a small saucepan over medium heat. Stir in flour and cook, stirring, until all the flour is incorporated into the butter, about 3 minutes. Season with 1/4 teaspoon herbes de Provence, black pepper, and any remaining salt. Slowly add milk, stirring constantly to avoid lumps.

Step 5

Continue to cook and stir until mixture has thickened and bubbles break open (some cooks call this stage "making bullseyes"), 5 to 8 minutes. Remove from heat and stir in 1/2 of the shredded Gruyere. Allow sauce to cool slightly.

Step 6

If using purchased pie dough, open the pouch and place dough onto the prepared sheet pan. Slowly unroll the dough, pinching together to close any tears. Sprinkle bread crumbs over the dough.

Step 7

Blot tomatoes and squash with paper towels.

Step 8

Ladle sauce over the dough, leaving about a 1 1/2-inch border without sauce. Starting on the outside edge of the sauce, alternate slices of yellow squash, onion, zucchini, and tomato, repeating until all the sauce is covered by the vegetables. Sprinkle remaining herbes de Provence over the vegetables, and gently fold the crust over the outside edge of the vegetables.

Step 9

Brush beaten egg ontothe folded pie dough.

Step 10

Place galette in preheated oven and bake until the crust is brown and the vegetables are tender, 25 to 30 minutes.

Step 11

Sprinkle remaining shredded cheese on top of the warm galette and bake until cheese is melted, 3 to 5 more minutes.

Step 12

Allow to cool for 5 minutes. Garnish with fresh herbs and serve.