How to Make It
Preheat oven to 425°F. Stir together mozzarella, mayonnaise, chives, parsley, basil, mustard, and pepper in a small bowl; set aside.
Stir together flour, baking powder, sugar, baking soda, 6 tablespoons of the cornmeal, and 1 teaspoon of the salt in a large bowl. Cut in butter using a pastry blender or 2 knives until pea-size crumbs form. Drizzle in buttermilk; gently stir until a sticky dough forms, about 1 minute. Knead once or twice to gather dough together.
Dust a 12- x 17-inch piece of parchment paper with cornmeal. Place dough on parchment paper, and pat into a 5-inch circle. Dust a rolling pin with flour, and roll dough into a 13-inch circle (about 1/4-inch thick). Sprinkle remaining 2 tablespoons cornmeal over dough, leaving a 1 1/2-inch border. Arrange tomatoes, overlapping, in a concentric circle over cornmeal. Sprinkle with remaining 1/4 teaspoon salt; dollop evenly with mozzarella mixture. Fold dough border up and over tomato slices, pleating if necessary.
Transfer tart to a baking sheet. Bake in preheated oven until golden brown and puffed, about 20 minutes. Let stand about 15 minutes; slice and serve.