In a large bowl, mix together the flour and salt. Add the cold, cubed butter and mix with a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, until the dough forms a ball. Wrap in plastic wrap and refrigerate for 30 minutes.

While the dough chills, preheat your oven to 400°F (200°C).

Slice the heirloom tomatoes into thin rounds, sprinkle with salt, and let them drain in a colander for about 10 minutes.

On a lightly floured surface, roll out the dough into a rough circle, about ¼ inch thick, and transfer to a baking sheet lined with parchment paper.

Sprinkle the shredded mozzarella cheese over the crust, leaving about an inch border around the edge. Layer the sliced tomatoes on top, overlapping slightly. Season with oregano, salt, and pepper.

Gently fold the edges of the dough over the filling, pleating as you go to create a rustic design.

Bake in the preheated oven for about 35 minutes or until the crust is golden brown and the tomatoes are slightly caramelized.

Allow the galette to cool for a few minutes before garnishing with fresh basil leaves. Slice into wedges and serve warm.