In a small bowl, whisk together olive oil, balsamic vinegar, minced garlic, salt, and pepper.
Place chicken breasts in a baking dish and cover with marinade. Refrigerate at least 30 minutes, up to overnight.
Preheat the grill to medium-high heat.
Grill the chicken breasts for 5-7 minutes on each side, depending on the thickness, until internal temperature reads 165 degrees F.
Brush slices of bread with olive oil.
Grill each piece 1-2 minutes on each side.
To assemble sandwiches, place one chicken breast half on a slice of bread.
Top with mozzarella cheese, tomato slices, and basil leaves.
Drizzle with balsamic reduction and then top with a slice of bread.
Serve and enjoy!