Preheat the oven to 375°F.
Remove the stems from the mushroom caps.
Blend the stem in a blender. Set aside. Then blend the sausage in the same blender. Transfer in a small bowl.
Heat butter in a large skillet over medium heat. Add garlic, onion, and mushroom stem to the skillet. Stir fry until all the moisture disappears (about 5 minutes). Set aside to cool down.
In a large bowl, combine the mushroom mixture with cream cheese, bread crumbs, sausage, half of the parmesan cheese, parsley, salt, and pepper. Stir well until you have a thick mixture.
Using a small spoon, fill each mushroom cap with a generous amount of stuffing.
Arrange the mushroom caps on a baking sheet. Bake for 15 minutes in the preheated oven.
Then sprinkle with the remaining parmesan cheese on top of the mushroom cap, bake for another 3 minutes until the cheese is melted and the mushrooms are deliciously tender.
Serve while the mushrooms are still hot.