Use any combination of fresh mushrooms in this appetizer. The mushroom topping would also be good over pasta. Shaved Parmigiano-Reggiano cheese looks handsome, but you can also grate the cheese and stir it into the topping.
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.
Great recipe! The guests and my husband loved it. I didn't use quite as much red wine vinegar as was called for.