Preheat oven to 400°. Remove stems from mushrooms; set caps aside. Finely chop stems. In a large skillet, heat butter over medium heat. Add chopped mushrooms and onion; saute until liquid has evaporated, about 5 minutes. Remove from heat; set aside., Meanwhile, combine remaining ingredients; add mushroom mixture. Stuff firmly into mushroom caps. Bake, uncovered, in a greased 15x10x1-in. baking pan until tender, 15-18 minutes. Serve warm.