Preheat oven to 350 degrees. Spray a baking sheet with nonstick cooking spray.
Clean all the mushrooms, then very carefully remove stems. Place the mushroom caps in a bowl and set aside. Transfer the mushroom stems to a cutting board and chop very finely.
Melt butter in a large skillet, over a medium heat, then add finely chopped mushroom stems and onions to the skillet. Sautee until all moisture has evaporated, then add garlic and continue cooking until fragrant. Remove from heat and let cool.
Once mushroom mixture has cooled, transfer it to a medium sized bowl. Add cream cheese, parmesan, Italian seasoning, hot sauce, and 1 tablespoon of parsley to the bowl, mix until well combined. Carefully spoon mixture into mushroom caps, and place on prepared baking sheet.
In a small bowl combine 1 tablespoon of parmesan and 1 tablespoon of breadcrumbs, then sprinkle over stuffed mushrooms. Bake for 20 minutes.
Remove stuffed mushrooms from oven, and sprinkle with remaining parsley. Serve immediately or store in an airtight container for up to 3 days.