Preheat your grill or stovetop grill pan over medium-high heat.
Season the chicken breasts with salt and pepper on both sides.
Grill the chicken for about 6-7 minutes on each side or until cooked through.
In a bowl, combine diced tomatoes, basil, garlic, balsamic vinegar, and olive oil.
Stir the mixture gently and season with salt and pepper.
Remove the chicken from the grill and let it rest for a few minutes.
While the chicken rests, toast the bread slices until golden brown.
Place the grilled chicken on top of each toasted slice of bread.
Spoon the tomato mixture generously over the chicken.
Optional: sprinkle shredded mozzarella cheese on top and return to the grill for a minute to melt.