1Heat oven to 350°F. Carefully remove stems from mushrooms. Chop enough stems to measure 1/2 cup.
2In 10-inch skillet, melt butter over medium heat. Cook onion and broccoli in butter 2 minutes, stirring occasionally. Stir in mushroom stems. Cook about 2 minutes, stirring occasionally, until broccoli is crisp-tender. Cool slightly. Stir in bread crumbs, cheese, basil, oregano, garlic salt and pepper.
3Spoon vegetable mixture evenly into mushroom caps, mounding slightly. Place stuffed mushrooms in ungreased 15x10x1-inch pan. Garnish each with bell pepper pieces.
4Bake uncovered 12 to 15 minutes or until filling is light golden brown.