In a small saucepan, bring the balsamic vinegar to a high simmer over medium heat. Stir, then reduce the heat to low and simmer until the vinegar has thickened and reduced by half, 8 to 10 minutes. Set aside to cool.
Place the strawberries, tomatoes, mozzarella, avocado, and basil in a shallow bowl or platter. Drizzle with olive oil and season generously with salt and pepper. Gently toss. Drizzle with reduced balsamic. Serve immediately.