In a food processor, combine all-purpose flour and kosher salt, pulsing briefly to mix. Add cold, cubed butter and pulse until the mixture resembles coarse crumbs, about 10 seconds. Gradually mix in apple cider vinegar and ice water until the dough forms a shaggy ball. Wrap the dough in plastic wrap and chill in the refrigerator for at least 1 hour.

While the dough chills, slice the heirloom tomatoes and sprinkle them lightly with salt to draw out moisture. Allow the tomatoes to rest in a colander for about 30 minutes, which helps prevent a soggy crust. After draining, gently pat them dry with paper towels.

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

Roll out the chilled dough on a lightly floured surface into a 14–16 inch circle. Transfer the dough to the prepared baking sheet. Sprinkle shredded sharp cheddar and parmesan evenly over the surface, leaving a 2-inch border. Layer the garlic-rubbed tomatoes on top, fold the crust edges inward, and create a rustic look. Brush the exposed crust with the egg wash.

Carefully place the galette in the preheated oven and bake for approximately 45 minutes, or until the crust is golden brown and flaky.

Garnish your Heirloom Tomato Galette with fresh thyme, chives, or basil before serving.