In a large bowl, whisk together the flour, salt, and sugar. Add the cold cubed butter and use your fingertips or a pastry cutter to work the butter into the flour until the mixture resembles coarse crumbles with some pea-sized butter chunks remaining — those chunks are what create flakiness.

Drizzle in the ice water one tablespoon at a time, tossing gently with a fork after each addition, until the dough just comes together when you squeeze a handful. You may need up to 5 tablespoons total. Do not overwork.

Turn the dough onto a lightly floured surface, gently shape into a disk, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes (or up to 3 days).

Arrange the tomato slices in a single layer on a paper-towel-lined baking sheet or in a colander set over a bowl. Sprinkle evenly with 1 teaspoon kosher salt and let sit for 20 to 30 minutes to draw out excess moisture.

After resting, blot the tomato slices thoroughly on both sides with fresh paper towels. This step is essential for preventing a soggy crust.

In a medium bowl, stir together the ricotta, grated Parmesan, minced garlic, 1 tablespoon olive oil, black pepper, and red pepper flakes if using. Taste and adjust seasoning as needed. The mixture should be creamy, well-seasoned, and spreadable.

Preheat your oven to 400°F (205°C). If desired, place an empty rimmed baking sheet in the oven while it preheats. Line a separate 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 and about ⅛-inch thick. Do not worry about perfect edges — rustic is the whole point. Transfer the rolled dough to your parchment-lined baking sheet.

Spread the ricotta mixture evenly over the center of the dough, leaving a 2-inch border all around.

Arrange the drained, patted-dry tomato slices over the ricotta in overlapping layers or a decorative pattern, using different colors for visual impact. Leave the 2-inch dough border bare.

Drizzle a thin thread of olive oil over the tomatoes. Fold the bare dough border up and over the edge of the filling, pleating it loosely as you go all the way around to contain the filling. Press gently to help the folds hold.

Brush the folded crust edges generously with the beaten egg wash. Sprinkle the crust with flaky sea salt.

Carefully slide the galette (on the parchment) onto the preheated baking sheet in the oven, or place on the room-temperature lined sheet if you skipped the preheat step.

Bake at 400°F for 35 to 42 minutes, until the crust is deep golden brown, the edges are crisp, and the tomatoes are slightly caramelized and bubbling. If the edges are browning too fast, tent loosely with foil for the last 10 minutes.

Remove from the oven and let the galette rest on the baking sheet for at least 10 minutes before cutting — this helps the filling set so it slices cleanly.

Scatter the torn fresh basil leaves generously over the warm galette. Finish with an optional final drizzle of your best extra-virgin olive oil. Slice into wedges and serve warm or at room temperature.