Preheat oven to 400F and slice tomatoes about 1/2 inch thick. Lay them out on a baking tray and roast for 30 minutes until slightly dehydrated.
While the tomatoes roast, make the dough by pulsing all the ingredients but the basil in the food processor. Once it comes together, remove from processor and knead in the chopped basil. Let rest for 1 hour.
To form galette, roll out dough over floored surface and then place on a lined baking tray. Sprinkle the center of it with cornmeal. Arrange the roasted tomatoes and mozzarella on top and dot with butter. Crimp the edges over and brush egg wash over it. Sprinkle with salt and pepper and bake for 30 minutes, or until golden brown. Let cool for ten minutes before cutting and serving.