Preheat the oven to 350 degrees Farenheit. In a medium skillet over medium heat, sauteé garlic and onions until translucent and very fragrant.
Add sausage to the medium skillet to join the onions and garlic. Break the sausage up while cooking to allow equal it to cook evenly. Once the sausage has been fully cooked, transfer to bowl and set aside to cool.
Stir in the parmesan cheese, mayonnaise, salt, pepper, and fresh parsley. Combine the ingredients evenly.Break off the stems of each mushroom. With a small spoon, *gently* scrape out the gills of the mushrooms and any additional stems. Repeat this pattern until all mushrooms have been scraped.
On a baking sheet, gently place each scraped mushroom cap side down. With a scoop *see notes section*, scoop the sausage mixture into each mushroom. Pack evenly to to prevent the mushrooms from tipping over.
Once all of the mushrooms have been stuffed with the sausage mixture, bake the mushrooms for twelve minutes. Garnish with additional fresh parsley.