Step 1: In a mixing bowl, combine the flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs.

Step 2: Stir in the sour cream and fresh lemon juice, then gradually add the cold water until a dough ball forms.

Step 3: Wrap the dough tightly in plastic wrap and refrigerate for 30 minutes.

Step 4: While the dough chills, slice the tomatoes and arrange them on a rack or paper towel-lined surface. Sprinkle lightly with salt and let them sit for 15 minutes to draw out excess moisture. Pat dry with paper towels.

Step 5: In a separate bowl, mix together the Parmesan, mozzarella, ricotta, cottage cheese, garlic powder, and fresh basil.

Step 6: Preheat the oven to 400°F.

Step 7: Roll the chilled dough into a circle about 14 inches across and roughly 1/8 inch thick.

Step 8: Spread the cheese mixture over the dough, leaving a 1 to 1½-inch border around the edge.

Step 9: Arrange the sliced tomatoes evenly over the cheese filling.

Step 10: Fold and pleat the edges of the dough over the filling, leaving the center exposed.

Step 11: Brush the exposed crust with the beaten egg.

Step 12: Bake for 30–35 minutes, or until the crust is golden brown and cooked through. If the edges begin browning too quickly, loosely tent with aluminum foil for the remainder of the bake time.

Allow the galette to cool for several minutes before slicing and serving.