In a large bowl, combine flour, salt, and 1/2 teaspoon oregano. Cut the butter into small pieces using a large, sharp knife, then add the butter to the flour. Using a pastry cutter (or two sharp knives) continue to cut the butter into smaller and smaller pieces until they are pebble sized. Add in 6 tablespoons of water, and with a rubber spatula, combine until the dough comes together into a ball. Wrap tightly in plastic wrap and refrigerate for 30 minutes. Preheat the oven to 375 degrees.
Roll out dough on a lightly floured surface, use a dab of water to repair any large cracks. Transfer to a parchment lined baking sheet. Add a layer of cheese slices and then tomatoes, leaving a one inch space around the edges. Sprinkle the tomatoes with garlic, salt, pepper and the remaining oregano. Bring the edges of the dough into the center. Brush the crust with the egg wash, and bake for 35-45 minutes until the crust turns golden brown.