Remove gills from undersides of mushrooms with a spoon. Place mushrooms on a baking sheet coated with cooking spray, underside up. Coat mushrooms with cooking spray. Drizzle 1/2 teaspoon olive oil over each mushroom. Bake, uncovered, at 400° for 8 minutes or until tender. Remove from oven. Set aside, and keep warm.
Cook ground round, basil, and fennel in a nonstick skillet over medium heat until beef is browned, stirring until it crumbles. Drain well.
Spoon pizza sauce evenly over mushrooms; top evenly with beef mixture and cheese. Broil 2 minutes or until cheese melts.