Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper and set aside.
In a skillet, cook the Italian sausage over medium heat until browned and cooked through, breaking it up into crumbles as it cooks.
Add the diced onion and minced garlic to the skillet with the cooked sausage, and cook for an additional 2-3 minutes, or until the onion is softened.
Remove the skillet from heat and transfer the sausage mixture to a mixing bowl.
Add the grated Parmesan cheese, breadcrumbs, chopped fresh parsley, dried oregano, salt, and black pepper to the mixing bowl. Stir until well combined.
Stuff each mushroom cap with the sausage mixture, pressing down gently to pack it in.
Arrange the stuffed mushrooms on the prepared baking sheet.
Drizzle olive oil over the stuffed mushrooms, then sprinkle shredded mozzarella cheese over the top.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the cheese is melted and golden brown.
Remove from the oven and let the stuffed mushrooms cool for a few minutes before serving.
