Preheat the oven to 200 degrees C. With a spoon, scrape out and discard the black gills from the mushrooms. Coat a baking tray with cooking spray. Place the mushrooms, hollow-side down, in the tray. Cover with a piece of foil and bake for about 5 minutes or until the mushrooms are tender. Remove the mushrooms from the baking tray and drain, hollow-side down, on paper towels. Leave the oven on.
Meanwhile, combine the beans, water, basil, Parmesan and pepper in a medium bowl. Mash until smooth using a potato masher or large fork.
Return the mushrooms, hollow-side up, to the baking tray. Spoon the bean mixture into the mushroom hollows and bake for 5 minutes.
Top with the mozzarella and tomatoes then bake for a further 5 minutes or until the cheese has melted and the bean mixture is hot.