Season the chicken. Pat the chicken breasts dry with paper towels. Season both sides with salt, pepper, and Italian seasoning, then place them in a single layer in the slow cooker.
Build the sauce. Sprinkle the minced garlic evenly over the chicken. Pour the drained diced tomatoes and balsamic vinaigrette over the top, and add the optional teaspoon of sugar to balance acidity.
Slow cook. Cover and cook on LOW for 6 hours or HIGH for 3 hours, until the chicken reaches an internal temperature of 165°F and shreds easily with a fork.
Melt the cheese. Sprinkle the shredded mozzarella evenly over the chicken. Cover and let melt for about 10 minutes — the residual heat is enough.
Make the bruschetta topping. While the cheese melts, combine the halved cherry tomatoes, chopped basil, olive oil, balsamic vinaigrette, and a pinch of salt and pepper in a small bowl. Toss gently to combine.
Serve. Spoon the fresh bruschetta topping generously over the cheesy chicken just before serving. Serve over pasta, polenta, rice, or with crusty bread.