1. Cut tomatoes into 1/2-inch-thick slices; sprinkle with salt and pepper. Let stand 10 minutes.
2. Combine cornmeal mix and panko in a shallow dish or pie plate. Place flour in a second shallow dish or pie plate. Whisk egg whites in a medium bowl until foamy. Dredge tomato slices in flour, shaking off excess. Dip in egg whites, and dredge in cornmeal mixture.
3. Cook half of tomato slices in 1 1/2 Tbsp. hot oil in a nonstick skillet over medium heat 4 to 5 minutes on each side or until golden brown. Season with salt to taste. Place on a wire rack in a jelly-roll pan, and keep warm in a 225° oven. Repeat procedure with remaining tomato slices and oil.