CRUST: In a large bowl, combine flour and salt. Cut in butter with a pastry blender or fork until it resembles coarse meal. Using a wood spoon, stir in ice cold water, parmesan cheese, oregano and thyme leaves, until well combined. With floured hands, divide the dough in half and shape into 2 disks. Wrap one disk in plastic refrigerate or freeze to use later.
On a well-floured surface, roll out the other disk into a 13-inch round. Transfer to a baking sheet lined with parchment paper. Refrigerate while you prepare the filling.
FILLING: Place the sliced tomatoes on a baking sheet lined with paper towels. Let stand so that the paper towels absorb the moisture, at least 30 minutes.
Heat olive oil in a large sauté pan over medium heat. Add the onions and cook for 8-10 minutes until softened and translucent. Add garlic and sauté for 30 seconds more. Remove from heat and allow to cool slightly.
In a small bowl, combine ricotta, mozzarella, lemon juice, basil, salt, and pepper. Add cooled onions and garlic mixture; stir until combined.
TO ASSEMBLE: Using an offset spatula, spread a thin layer of filling gently over the pastry, leaving a 1-1/2 -inch border around the edges. Pat the tomatoes dry with paper towels and place in a circular pattern around the outer edges directly over the ricotta filling, overlapping them slightly. Repeat with remaining tomatoes in the middle of the galette.
Fold the edges over, crimping and overlapping the edges. Brush the edges with egg wash.
Bake at 4oo degrees until the galette begins to brown, about 15 minutes. Reduce heat to 375 degrees and bake until golden brown on the top and bottom, about 20-25 minutes more.
Cool for 5 minutes on a wire rack. Serve warm garnished with fresh basil, Enjoy!