In a large bowl, combine the all-purpose flour and salt. Add the chilled, cubed unsalted butter to the flour mixture, and use a pastry cutter or your fingers to combine until the mixture resembles coarse crumbs.

Slowly add in ice water, 1 tablespoon at a time, mixing until the dough comes together. Wrap the dough in plastic and refrigerate for at least 30 minutes.

While your dough is chilling, preheat the oven to 375°F (190°C).

Thinly slice your heirloom tomatoes and gently pat them dry with a paper towel to remove excess moisture. Sprinkle them with salt and let them sit for about 10 minutes.

On a floured surface, roll out the chilled dough into a circle about 12 inches in diameter. Transfer the dough to a parchment-lined baking sheet.

Drizzle the olive oil over the crust. Evenly sprinkle the shredded mozzarella cheese over the dough, leaving a 2-inch border.

Layer the sliced heirloom tomatoes over the cheese, and sprinkle with chopped basil, salt, and pepper to taste.

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

Bake the galette in the preheated oven for 35-40 minutes, until the crust is golden brown and the cheese is melted and bubbly.

Allow the galette to cool slightly before slicing. Serve warm or at room temperature.