How to Make It
Combine 1 cup water, 1 cup vinegar, sugar, and 1/2 teaspoon salt in a medium saucepan; bring to a boil. Add tomatoes; cook 2 minutes. Remove from heat; let stand 15 minutes, stirring occasionally. Drain tomatoes; pat dry.
Combine 5 tablespoons buttermilk and next 5 ingredients (through garlic), stirring with a whisk. Stir in 1/4 teaspoon pepper.
Heat a large skillet over medium heat. Add panko to pan; cook 2 minutes or until toasted, stirring frequently. Remove from heat; stir in masa harina, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon pepper. Place panko mixture in a shallow dish. Combine remaining 2 tablespoons buttermilk, egg, and egg white, stirring with a whisk. Place flour in another shallow dish. Dredge tomato slices in flour; dip in egg mixture, and dredge in panko mixture, turning to coat.
Heat a large nonstick skillet over medium-high heat. Add 1 tablespoon oil to pan; swirl to coat. Add half of tomatoes; cook 4 minutes. Coat tops of tomatoes with cooking spray. Turn; add 1 1/2 teaspoons oil to pan. Cook 4 minutes or until golden. Repeat procedure with remaining 1 1/2 tablespoons oil, tomatoes, and cooking spray. Serve with sauce.