Chop mushroom stems. In microwaveable pie plate using a fork, combine mushroom stems, sausage, Italian seasoning, garlic, Parmesan cheese, ½ cup Mozzarella cheese and green onions. Cover with paper towel.
Microwave on HIGH (100% power) for 1 minute. Arrange mushroom caps on paper towel lined microwaveable pie plate. Spoon about 1 tablespoon sausage mixture into each cap. Sprinkle the remaining Mozzarella over the stuffed mushrooms. Microwave on HIGH (100% power) uncovered, until cheese melts, about 30 seconds.