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