Combine all-purpose flour and sea salt in a mixing bowl. Cut in very cold butter until the mixture resembles coarse crumbs. Gradually mix in ice water, then shape into a disc, wrap in plastic, and refrigerate for about 1 hour.

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

Roll the chilled dough into a rough 12-inch circle on a floured surface and transfer it to the prepared baking sheet.

Spread mascarpone cheese over the dough, leaving a 1-inch border. Arrange sliced heirloom tomatoes on top and sprinkle with sumac, salt, and pepper.

Fold the edges of the dough over the filling to create pleats and tuck them in gently.

Bake for 30-35 minutes until the crust is golden brown and the cheese filling begins to bubble.

Let the galette cool slightly for 5-10 minutes before serving warm or at room temperature.