Preheat oven to 350 degrees. Remove stems from cap; reserve caps. Dice stems.
Melt butter in a skillet over medium-high heat. Add diced stems and onion; saute 10 minutes until liquid evaporates. In a bowl, combine mushroom mixture, tomatoes, barley, oregano and cheese. Spoon mixture evenly into mushroom caps.
Place mushrooms on baking sheet. Sprinkle with parmesan cheese. Bake for 20 minutes or until tender.