Season chicken on both sides with oil, basil, oregano, salt and pepper.
Heat 2 teaspoons olive oil in a large oven-proof skillet over medium-high heat. Add the chicken and fry for 2 minutes on each side until golden. Remove to plate
Mix together the diced red onion, garlic, balsamic vinegar and brown sugar. Pour the mixture into the pan and simmer, stirring, 2-3 minutes.
Return chicken to pan and spoon sauce over it. Place pan in oven and roast until the chicken is cooked through (165 degrees), about 10-20 minutes depending on how thick the breasts are. Remove from oven.
Turn oven to broil.
Top each chicken with a slice of fresh mozzarella cheese. Broil for 2-3 minutes, or until the cheese has melted completely.
Serve immediately, topped with fresh sliced tomatoes and basil and spooning the pan sauce over the top.