Prepare the chicken marinade. Place all of the ingredients for the marinade into a jar. Cover the jar and shake until all of the ingredients are combined. 

Add the chicken breasts to a gallon-size Ziploc bag or a casserole dish and pour the marinade over the chicken. Be sure the chicken is completely coated in the marinade. Seal the bag and place the chicken in the fridge for at least 30 minutes or as long as overnight (the longer the better). 

While the chicken is marinating, prepare the bruschetta topping. Place the diced tomatoes in a sieve and sprinkle them with ¼ teaspoon salt. Mix the two ingredients together. Place the sieve over a large bowl and let the tomatoes sit for 10 minutes to drain excess liquid from the tomatoes. Discard the excess liquid and place the tomatoes in a large mixing bowl. 

Add the red onion, garlic, pepper, olive oil, balsamic vinegar, and basil to the bowl with the tomatoes and toss. Set aside for later.

Preheat the grill to 375ºF. 

Remove the chicken from the refrigerator and remove the chicken breasts from the marinade. Let excess marinade drip off of the chicken breasts. Do not discard the marinade. 

Place the chicken breasts over direct heat for 4 minutes, brush each chicken breast with excess marinade and then flip the chicken breast over and grill for another 4 minutes over direct heat. 

Move the chicken breasts over to indirect heat and grill them for an additional 10 minutes. 

When the chicken is almost fully cooked, around 155ºF - 160ºF, place thin slices of mozzarella cheese on top of each chicken breast. Once the chicken reaches 165ºF and the mozzarella cheese has melted, remove the chicken from the grill and let rest for 5 minutes.  

Serve chicken breast topped with a generous amount of bruschetta and a drizzle of balsamic reduction.