Wash mushrooms, remove stems, chop stems fine and reserve. Drain caps.
Melt butter in skillet; add onion, pepperoni, green pepper, garlic and chopped mushroom stems. Cook until vegetables are tender but not brown.
Add cracker crumbs, cheese, parsley, seasoned salt and oregeno and mix well. Stir in chicken broth.
Spoon stuffing into mushroom caps, rounding tops. Place caps in shallow baking pan with 1/4 inch of water covering bottom of pan. Bake uncovered 325 degrees for 25 minutes.