Distribute breadcrumbs (or semolina) over the center of the dough, leaving a 2-inch border. Evenly spread the filling over the breadcrumbs (or semolina). Lift the edge of the dough, folding it over and partly covering the filling and making slightly overlapping pleats as you work your way around. Gently press on the top and sides of the dough to ensure a good seal. Dot the exposed filling with butter. Whisk egg and water in a small bowl and brush over the dough; sprinkle with turbinado sugar. Chill the galette for 30 minutes.