Preheat the oven to 375°F (190°C).
Clean the mushrooms and remove the stems.
In a skillet, brown the Italian sausage over medium heat, breaking it apart with a spatula.
Add the chopped mushroom stems and garlic to the skillet, cooking until softened.
In a mixing bowl, combine the cooked sausage mixture with breadcrumbs, mozzarella, Parmesan, parsley, salt, and pepper.
Stuff each mushroom cap generously with the filling mixture, pressing down lightly.
Place the stuffed mushrooms on a baking sheet lined with parchment paper.
Drizzle olive oil over the stuffed mushrooms before baking.
Bake in the preheated oven for 20-25 minutes until the tops are golden and the mushrooms are tender.
Remove from the oven and let cool slightly before serving.