Whisk together first 6 ingredients, ½ cup of the buttermilk, 1 teaspoon of the salt, and ½ teaspoon of the black pepper. Cover; chill buttermilk dressing until ready to serve.

Pour vegetable oil to a depth of 3 inches in a large cast-iron Dutch oven; heat over high to 350°F. Whisk together egg and remaining ½ cup buttermilk in a shallow dish. Combine cornmeal, cayenne, ½ cup of the flour, 1 teaspoon of the salt, and ½ teaspoon of the black pepper in a second shallow dish. Combine remaining ½ cup flour, ½ teaspoon salt, and ½ teaspoon black pepper in a third shallow dish. Working with 1 slice at a time, dredge tomato in flour mixture, then in egg mixture, and then in cornmeal-flour mixture.

Fry tomato slices, in 2 batches, in hot oil until browned, about 3 to 4 minutes total, turning slices halfway through frying. Drain on paper towels.

Combine lettuces and red onion in a large bowl. Toss lettuce mixture with ¼ cup of the buttermilk dressing, reserving the rest. Divide salad among 4 plates, top each with 3 fried green tomatoes. Drizzle evenly with remaining dressing.