Remove stems from mushrooms. Finely chop enough stems to measure 1/4 cup. Discard remaining stems or refrigerate for another use.
Melt butter in small skillet on medium heat. Add chopped stems, onions and peppers; cook and stir 5 min. or until tender. Stir in cracker crumbs, cheese and seasoning; spoon into mushroom caps. Place on baking sheet.