Ingredients
12 cocktail tomatoes (about 12 ounces total), halved
Kosher salt and freshly ground pepper
1 3/4 cups plus 1 tablespoon unbleached all-purpose flour, and more for dusting
1 tablespoon sugar
1 stick cold, unsalted butter, cut into 1/2-inch cubes, plus 2 tablespoons
1 1/2 ounces Parmigiano-Reggiano, grated (1/2 cup lightly packed)
2 1/2 ounces Gruyère, shredded (1 1/4 cups)
3 to 5 tablespoons ice-cold water
1 clove garlic, minced (1 teaspoon)
1 teaspoon fresh thyme leaves
3/4 cup whole milk
1 tablespoon Dijon mustard
1 cup fresh corn kernels (from 2 ears)
1 medium zucchini, cut into 1/4-inch rounds (1 1/3 cups)
Flaky sea salt, such as Jacobsen
1 large egg, beaten
Preparation
Preheat oven to 375 degrees. Arrange tomatoes, cut-sides up, on a parchment-lined rimmed baking sheet; season with kosher salt. Roast until tomatoes shrink and collapse slightly, about 1 hour. Let cool completely.
Meanwhile, pulse 1 3/4 cups flour, sugar, 3/4 teaspoon kosher salt, and 1/4 teaspoon pepper in a food processor until combined. Add cubed butter, Parmigiano-Reggiano, and 1/2 cup Gruyère; pulse until mixture resembles coarse meal, about 10 seconds. Drizzle 3 tablespoons ice-cold water evenly over mixture; pulse until it holds together when pressed between your fingers. (Dough should not be wet or sticky; if too dry, pulse in more water, 1 tablespoon at a time.) Shape into a disk, wrap in plastic, and refrigerate until firm, about 1 hour.
Melt remaining 2 tablespoons butter in a saucepan over medium heat; bring to a simmer. Add garlic and thyme; cook until fragrant, about 1 minute. Stir in remaining 1 tablespoon flour; cook until nutty and golden, about 2 minutes more. Add milk, return to a simmer, and cook, stirring occasionally, until reduced and thickened slightly, about 5 minutes. Remove from heat. Add remaining 3/4 cup Gruyère and mustard; whisk until smooth. Stir in 1/2 teaspoon kosher salt and 1/4 teaspoon pepper. Let cool completely, about 30 minutes.
On a lightly floured sheet of parchment, roll out dough to a 14-inch round. Spread cheese sauce evenly over dough, leaving a 2-inch border. Scatter half of corn evenly over sauce, then scatter zucchini evenly over corn. Top with roasted tomatoes, cut-sides up, and sprinkle with remaining corn. Season with flaky salt and pepper. Fold edges of dough up over vegetables, then brush dough with beaten egg. Transfer galette on parchment to a baking sheet.
Bake until crust is golden brown and set on bottom, and vegetables are tender and golden brown in places, 1 hour, 10 minutes to 1 hour, 20 minutes (if crust is browning too quickly, tent with foil). Transfer on parchment to a wire rack. Let cool until warm but no longer hot (about 30 minutes), or completely, before slicing and serving.
