Pop off the stem of each mushroom and place on a baking sheet.
Preheat oven to 350 degrees F.
In a medium skillet, cook Italian sausage, breaking it up into small pieces as it cooks.
Remove sausage from skillet but reserve the grease for cooking the onion.
Using the grease from the sausage, cook the onion until softened, about 5-7 minutes.
Add garlic, Italian seasoning, and salt, and cook one more minute.
In a large bowl, combine sausage, onion mixture, cream cheese, and Parmesan cheese and mix well until combined.
Stuff each mushroom with a tablespoon or two of the mixture, trying to make a rounded mound on top to get as much into each mushroom.
Bake in the preheated oven for 20-25 minutes, until golden.
Serve topped with chopped parsley.