In a large bowl, whisk together the flour, salt, and sugar. Add the cold butter cubes and use your fingertips to work the butter into the flour until the mixture resembles coarse, shaggy crumbs with some pea-sized butter pieces remaining — these layers are what create flakiness.

Combine the ice water and apple cider vinegar. Drizzle 3 tablespoons over the flour mixture and use a fork or your hands to bring the dough together, adding the remaining tablespoon of water only if needed. The dough should hold together when pinched but not feel wet or sticky.

Shape the dough into a flat disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour and up to 3 days. Do not skip this step — cold dough is essential for a flaky crust.

Slice the heirloom tomatoes approximately 1/4-inch thick and arrange them in a single layer on a wire rack set over a baking sheet, or on paper towels. Sprinkle generously with 1 teaspoon kosher salt and let them drain for at least 30 to 45 minutes.

Pat the tomatoes thoroughly dry with fresh paper towels before using. This step removes excess moisture and is the key to a crisp, non-soggy crust.

In a medium bowl, stir together the ricotta, grated Parmesan, minced garlic, thyme, basil, lemon zest, salt, and black pepper until well combined. Taste and adjust seasoning as needed. Set aside.

Preheat the oven to 400°F (205°C). Line a large rimmed baking sheet with parchment paper.

On a lightly floured surface, roll the chilled dough into a rough circle approximately 13 to 14 inches in diameter. The edges do not need to be perfect — rustic and irregular is the goal. Carefully transfer the dough to the prepared parchment-lined baking sheet.

Spread the herbed ricotta mixture evenly over the dough, leaving a 2-inch border around the edges.

Arrange the drained heirloom tomato slices over the ricotta in overlapping, concentric layers. Alternate colors and varieties for maximum visual impact. Drizzle with 1 tablespoon of olive oil and sprinkle with red pepper flakes if using.

Fold the pastry border up and over the outer edge of the tomatoes, pleating as you go to create a rustic rim. The folds do not need to be uniform or neat. Press the folds gently to help them hold.

Brush the folded pastry edges with the egg wash. Drizzle the remaining tablespoon of olive oil over the tomatoes. Sprinkle the entire galette with flaky sea salt and cracked black pepper. Refrigerate the assembled galette for 15 minutes while the oven finishes preheating.

Bake on the center rack at 400°F for 40 to 50 minutes, until the pastry is deeply golden brown and caramelized at the edges and the tomatoes have softened and begun to blister.

Remove from the oven and let the galette cool on the baking sheet for at least 10 to 15 minutes before slicing — this allows the filling to set. Top with fresh basil leaves and an optional drizzle of good olive oil or balsamic glaze. Slice and serve warm or at room temperature.