Clean mushrooms, remove stems. Reserve 8 stems, mince to add to filling. Gently scrape out some of the innards of each cap.

Arrange caps top side down in a 9x13 baking dish. Pour 1/2 cup water around caps. Set aside. Preheat oven to 375°F.

Heat a large skillet over medium heat. Add sausage. Cook until browned breaking up to a fine crumble. Sprinkle with thyme and add minced mushroom stems about half way through cooking.

Drain fat off sausage. Return to skillet over low heat. Stir in cream cheese and mayo until well blended. Add green onions. Gently stir to combine. Taste, add salt & pepper if desired. Let stuffing cool 15 minutes.

Once cool, transfer to a gallon size ziploc bag. Squeeze out air and seal. Cut off one lower corner. Now you have a homemade piping bag.

Squeeze stuffing into each cap, distributing as evenly as possible to practically overflow each cap.

Top mushrooms with shredded cheese. Cover dish with foil. Bake covered 20 minutes. Remove foil. Bake 15 minutes more. Garnish with additional green onions for serving if desired. Enjoy!