In a small bowl, combine the tomatoes, balsamic vinegar, red onion, fresh basil, garlic, and salt. Let chill in the fridge.
Season chicken with salt and pepper. If I have time I marinate the chicken in a little balsamic vinegar to give it more flavor. You can cook this on the stove or on the grill. In a large skillet, heat the olive oil over medium heat (if grilling skip the olive oil and grease the grates). Cook the chicken breasts about 4-6 minutes per side until done. Add a 1 tablespoon of basil pesto on top of each chicken breasts and place a slice of mozzarella on top. Cover to melt the cheese. Remove from heat.
With a slotted spoon, add the tomato mixture on top (you don't need the liquid). Drizzle with the balsamic glaze and sprinkle with Parmesan if desired.
Notes
The chicken breasts should be thin. If they are thick (like the Costco frozen chicken breasts) slice them in half lengthwise. For this recipe, I use two Costco chicken breasts halved. You can also use tenderloins if desired.