Preheat oven to 200°F. Place a cooling rack over a baking sheet.

In a bowl, combine the tomatoes, basil, red onion, 1 tablespoon of the olive oil and vinegar. Season with the salt and pepper.

Place the remaining 3 tablespoons of oil in a large sauté pan and heat over medium heat.

If using 4 smaller chicken breasts, place each breast between 2 sheets of plastic wrap and pound lightly with a meat mallet to until they are about 3/8” thick.

Place the eggs in a shallow dish, and the panko breadcrumbs in a separate shallow dish. Dip each piece of chicken into the eggs to coat, then into the panko. Use your hands to press the breadcrumbs into the chicken, if needed. Cook in 2 batches in the hot olive oil until crisp on both sides and cooked through, about 5 minutes per side. Remove and place on the baking sheet and place in the oven to keep warm.

When all of the chicken has been cooked, return all 4 pieces of chicken to the pan and top with the mozzarella. Cover the pan and cook just until the cheese is melted, 3-5 minutes.

Transfer the chicken to a serving plate and top with the tomatoes.