Cream together cream cheese, Parmesan cheese, and onions.
Cook sausage.
Once fully cooked, add in mushroom stems and cook until tender.
Pour sausage and mushrooms into cheese and onion mixture. Mix well.
Spoon into mushroom tops and bake for 15 minutes until bubbly.
Remove from oven and allow to cool for 5 minutes prior to serving.
Notes
Storage Tips
To make ahead: Prep according to the recipe instructions, except for baking them, and place in an airtight container and refrigerate for up to one day. Remove and place on a lined baking sheet and bake as directed. To refrigerate: Stuffed mushrooms are best the day they are baked, but still good later too. If you have some left, place it in an airtight container in the refrigerator for up to 3 days. Reheat in the microwave, the oven or for a couple of minutes in the air fryer.