Preheat oven to 375°F (190°C). Clean mushrooms with a damp paper towel, remove stems, and finely chop the stems for the filling. Arrange mushroom caps on a parchment-lined baking sheet.
Heat olive oil in a skillet over medium heat. Add sausage and chopped mushroom stems. Cook until sausage is browned, about 5–6 minutes, breaking it up as it cooks. Add garlic and cook 30 seconds more. Drain excess fat if needed.
In a bowl, combine cooked sausage mixture with cream cheese, mozzarella, Parmesan, parsley, and Italian seasoning. Mix until well blended. Season with salt and pepper to taste.
Spoon filling into mushroom caps, mounding slightly. Sprinkle with extra Parmesan if you like.
Bake for 18–20 minutes, until mushrooms are tender and tops are golden brown. Garnish with extra parsley and serve warm.