Clean mushrooms. Remove stems and finely chop. Arrange bacon in a 1-quart round glass casserole. Cover and microwave on high 3 to 4 minutes, or until crisp. Remove and crumble. Pour off all but 1 tablespoon drippings. Add onion and chopped mushroom stems to drippings. Microwave on high 3 to 4 minutes, or until just tender. Stir in sour cream, Parmesan cheese and bread crumbs. Spoon about 2 teaspoons stuffing into each mushroom cap. Arrange in 2 (8-inch) round glass cake pans. Microwave one dish at a time on high for 3 to 4 minutes. Makes 30 appetizers.