In a large bowl, combine the cooked sausage mixture, cream cheese, Parmesan cheese, mozzarella cheese, parsley, Italian seasoning, salt, and pepper. Mix until well combined.
Spoon the filling generously into each mushroom cap, pressing down slightly to pack it in.
Place the stuffed mushrooms on a baking sheet lined with parchment paper. Drizzle a little olive oil over the stuffed mushrooms.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the tops are golden brown.
Remove from the oven and allow to cool slightly before serving.
Notes
Serve warm for best flavor.
These mushrooms can be made ahead of time and baked just before serving.