Preheat oven to 400 degrees F.
Roll out the puff pastry dough on a lightly floured surface to form a rectangle less than 1/4-inch thick. Place on top of a cookie sheet, lined with parchment paper. Brush dough with egg wash. Bake for 10 minutes.
Cover baked dough with pesto. Add sliced tomatoes and mozzarella on top. Bake for another 5 minutes or until cheese melts.