Remove brown gills from the undersides of mushrooms using a spoon; discard gills. Place mushrooms, gill sides down, on a foil-lined baking sheet coated with cooking spray. Broil the mushrooms for 5 minutes.
While mushrooms broil, combine the tomatoes, panko breadcrumbs, cheese, and chives.
Turn mushrooms over, and sprinkle evenly with salt and pepper. Divide tomato mixture evenly among mushrooms. Broil 5 minutes or until cheese melts.