In the work bowl of a food processor, place flour, cornmeal, 1/4 teaspoon salt, and 1/8 teaspoon pepper; pulse until combined. Add cold butter, and pulse until mixture is crumbly. With processor running, add cold buttermilk in a slow, steady stream just until dough comes together. Turn out dough onto a very lightly floured surface. Shape into a disk, and wrap in plastic wrap. Refrigerate for at least 1 hour.
On a rimmed baking sheet lined with paper towels, place tomato slices. Sprinkle with 1 1/2 teaspoons salt. Let stand for 15 minutes. Pat dry with paper towels.
In a large skillet, heat oil over medium heat. Add onion, and cook until tender and beginning to brown, about 10 minutes. Add garlic, and cook for 1 minute. Remove from heat.
Preheat oven to 400°.
Let dough stand at room temperature for 10 minutes. On a lightly floured surface, roll dough into a 12-inch circle.
Spray a 9-inch pie plate with baking spray with flour. Transfer dough to prepared plate, pressing into bottom and up sides. Fold edges under, and crimp, if desired. Refrigerate for 10 minutes.
In a medium bowl, stir together 1 1/2 cups mozzarella, goat cheese, mayonnaise, egg, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper.
Place onion mixture in bottom of prepared crust. Top with half of cheese mixture; top with half of tomato slices. Sprinkle with 1 tablespoon basil. Spread remaining cheese mixture on top, and top with remaining tomato slices.
Bake until golden brown and center is set, 35 to 40 minutes, covering crust with foil after 20 to 25 minutes of baking to prevent excess browning. Sprinkle with remaining 1/2 cup mozzarella, and bake until cheese is melted, about 3 minutes more. Serve warm or at room temperature. Sprinkle with remaining 1 tablespoon basil. Garnish with pepper, if desired. Cover and refrigerate for up to 2 days.
*We used Blue Plate Mayonnaise.
This recipe is courtesy of Louisiana Cookin' magazine and was tested by the Louisiana Cookin' test kitchen. Blue Plate® is the official mayonnaise of Louisiana Cookin'. For more recipes from Louisiana Cookin', click here.