Pour balsamic vinegar into a small saucepan then bring to a boil. Turn heat down to medium then simmer and reduce until vinegar is thick enough to coat a spoon, about 20 minutes. (Vinegar will continue to thicken while cooling.) Set aside to cool.
Place oven rack in top position then preheat broiler. Brush chicken breasts with extra virgin olive oil then season with salt and pepper on both sides. Brush a large skillet over medium-high heat with oil then add chicken breasts and cook for 4-5 minutes a side, or until no longer pink in the center.
Transfer chicken to a foil-lined, nonstick-sprayed baking sheet then top with 2 Tablespoons bruschetta mixture, 3-4 slices avocado, and 1 oz mozzarella each. Broil until cheese has melted then serve over spinach or cooked pasta, and drizzle with the balsamic vinegar reduction.
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