Wash mushrooms and drain; remove stems and chop, set caps aside. Melt butter in a skillet; add onion and sauté 5 minutes.
Add pepperoni, green pepper, garlic, and mushroom stems. Cook 10 minutes, until tender.
Add crumbs, cheese, parsley, salt, oregano, and pepper. Mix well; stir in broth. Spoon stuffing into mushroom caps, rounding the tops. Place in a shallow pan with about 1/4 inch water. Bake 25 minutes. Serve immediately.