Preheat oven to 400 F. and lightly grease baking dish. Chop reserved mushroom stems fine. In a heavy skillet cook chopped stems and scallions in butter over moderately low heat, stirring, until stems are very tender. Stir in tomatoes, bread crumbs, 1 tablespoon of cheese, and salt and pepper to taste. Divide mixture among mushroom caps, mounding it slightly, and sprinkle with remaining tablespoon cheese. Arrange mushrooms in prepared baking dish and bake 12 minutes. Serves 2.