Preheat oven to 400. Remove stems. Set aside caps. Finely chop stems.

Cook bacon until crisp. Drain, crumble, set aside.

In saucepan over med. heat, melt butter. Cook and stir stems and onions until tender, about 15 min. Remove from heat.

In med. bowl, stir together onions, stems, bacon and 1/2 cup cheese. Mix well. Spoon into mushroom caps.

Bake for 15 min. Remove from oven. Top with remaining cheese and sprinkle with parsley.