In a medium mixing bowl, whisk together the all-purpose flour and salt until well combined. Add the cold, cubed butter, and use your fingers or a pastry cutter to work the butter into the flour until the mixture resembles coarse crumbs with pea-sized butter pieces.
Gradually stir in the ice water, one tablespoon at a time, mixing gently until the dough begins to hold together. Shape the dough into a disk, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes to chill and firm up.
Preheat your oven to 400°F (200°C).
Once the dough has chilled, lightly flour a clean work surface. Roll out the dough into a rough circle, about 12 inches in diameter, ensuring it is of even thickness. Transfer the rolled dough to a baking sheet that has been dusted with cornmeal to prevent sticking.
In a separate bowl, combine the shredded mozzarella, ricotta cheese, olive oil, and chopped basil. Season the mixture with salt and freshly cracked pepper to taste. Spread this cheesy basil mixture evenly over the center of the dough, leaving a 2-inch border around the edges for folding.
Layer the tomato slices artfully over the cheese mixture, slightly overlapping the slices for an appealing look.
Carefully fold the edges of the dough over the tomatoes, creating pleats as necessary for a rustic appearance and to encase the filling slightly.
In a small bowl, beat the egg to create an egg wash. Brush the egg wash over the exposed dough edges to give your galette a beautiful golden finish when baked.
Bake the galette in the preheated oven for 30-35 minutes, or until the crust is golden brown and crispy, and the tomatoes are tender.
Once baked, remove the galette from the oven and allow it to cool for about 5 minutes. If desired, garnish with extra fresh basil leaves for added flavor and color.
Notes
Serve warm on a rustic wooden cutting board, cut into wedges for easy sharing.