1. Heat oven to 375 degrees F.Biscuits:
2. Core tomato and slice in half. Squeeze out seeds and flesh. Finely dice; set on paper towel to dry slightly.
3. Place flour, cornmeal, cheese, baking powder, salt, pepper, baking soda and cayenne in a food processor. Add butter and pulse a few times until mixture looks like breadcrumbs. Add buttermilk and egg; pulse until mixture comes together in a ball. Transfer to work surface, and gently knead in diced tomato. If it is too wet, add a spoonful or two of flour. Pat dough into a 1/2-inch-thick circle. With floured biscuit cutter, cut into 8 rounds.
4. Bake biscuits at 375 degrees F for 25 minutes, until golden on top. Let cool.Topping:
5. In large bowl, combine tomatoes, basil, oil, garlic, vinegar, salt and pepper. To serve, heat broiler. Split biscuits in half. Place, cut side up, on foil-lined broiler pan. Broil 1 to 2 minutes or until lightly browned. Remove to platter; divide topping evenly among biscuits.