1. Remove stems from mushrooms; finely chop 1/4 cup stems.
2. Cook and stir chopped stems, onion and pepper in margarine in skillet over medium heat until tender. Remove from heat; stir in crumbs and basil.
3. Spoon crumb mixture loosely into mushroom caps; place on baking sheet. Bake at 400°F for 15 minutes or until hot.