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.
Note-Jeanine's recipe calls for 1/3 cup toasted pecans, but I left them out. If you don't want to reduce your own balsamic, you can use DeLallo's balsamic glaze. It is my favorite!