Clean mushrooms and separate caps and stems. Arrange caps hollow side up in a single layer in a 10 inch round baking dish. Set aside.
Dice mushroom stems finely. Combine with butter and green onion in a 1 qt. glass casserole. Microwave, uncovered at power level high for 4 to 5 minutes, or until onion is tender, stirring twice. Add bread crumbs and parsley flakes, set aside.
Cover mushroom caps and microwave on high for 2 1/2 to 4 minutes, or until nearly cooked. Stuff each cap with some of the bread crumb mixture. Cover and microwave on high for 2 1/2 minutes, or until hot.