Preheat your oven to 400 degrees F. Spray a rimmed baking sheet with cooking spray.
Place mushrooms on baking sheet and spread 1 tablespoon pesto in each mushroom cap. Top evenly with mozzarella, then tomatoes.
Bake for 13-15 minutes, or until cheese is melted and bubbly.
While pizzas are baking, prepare the balsamic reduction. Place balsamic vinegar in a small saucepan over medium-high heat. Bring to a boil and reduce heat to maintain a simmer. Simmer for approximately 8-10 minutes, or until desired consistency is met and reduced by about half.
Top pizzas with fresh basil and drizzle with balsamic reduction.