Preheat oven to 400° . Remove and chop mushroom stems.
Melt I Can't Believe It's Not Butter!® Spread in 12-inch skillet over medium-high heat and cook mushroom stems and onion, stirring occasionally, 5 minutes or until tender. Add garlic and cook 30 seconds. In medium bowl, pour mushroom mixture over bread crumbs. Stir in cheeses, parsley, vinegar and pepper.
Arrange mushroom caps on baking sheet, then sprinkle with salt. Evenly spoon mushroom mixture into mushroom caps. Bake 25 minutes or until mushrooms are tender and golden.