In a large bowl, combine the chicken breasts with garlic, balsamic vinegar, olive oil, Italian seasoning, salt, and pepper. Stir to coat. Cover and refrigerate for at least 30 minutes.

While chicken marinates, prepare the salsa by dicing avocados and halving tomatoes. Mix with basil, olive oil, lemon juice, salt, and pepper. Set aside in the fridge.

Preheat grill to medium-high. Grill the marinated chicken for 8-10 minutes per side until cooked through and juices run clear.

Remove from heat and top each grilled chicken breast with mozzarella. Serve topped with avocado tomato salsa.