In a medium bowl, combine the first 7 ingredients (through basil); set aside.
In a medium nonstick skillet, melt the butter over medium heat. Add the mushrooms, onions and garlic; cook 5 minutes or until tender, stirring frequently. Add the mushroom mixture to the tomato mixture; toss well to combine.
Spoon about 1 tablespoon of the mushroom/tomato mixture onto each bread slice. Sprinkle evenly with cheese. Serve immediately.