Preheat oven to 400°F. Remove and chop mushroom stems. In 12-inch skillet, melt butter over medium-high heat and cook mushroom stems and onion, stirring occasionally, 5 minutes or until tender. In medium bowl, pour mushroom mixture over bread crumbs. Stir in Boursin, parsley, salt and pepper. Evenly spoon mushroom mixture into mushroom caps; arrange on baking sheet. Bake 20 minutes or until mushrooms are tender and golden. Sprinkle with cheese.