Preheat the oven to 400 degrees and line a baking sheet with parchment paper.
Place the pie crust on the prepared sheet. In a bowl, toss the halved cherry tomatoes with Worcestershire sauce, hot sauce, celery salt, olive oil, and a few grinds of black pepper.
Arrange the sliced mozzarella over the center of the crust, leaving a 2-inch border. Spoon the seasoned tomatoes on top of the mozzarella.
Fold the crust edges over the filling, pleating as you go to create a rustic border. Brush the folded crust with the beaten egg.
Bake for 30 to 35 minutes until the crust is golden and the tomatoes have softened. Let the galette cool for 10 minutes, then scatter fresh basil leaves over the top before slicing.