Preheat oven to 350 °F.
In a skillet over medium heat, warm olive oil. Toss in sausages, casings removed. Break up as they cook. Add onion, pepper and garlic to pan. Continue cooking until meat is cooked through and vegetables are softened. Transfer to a medium bowl.
Sprinkle in breadcrumbs, 1/4 cup cheese and parsley. Combine ingredients.
Remove stems from mushrooms. Lightly drizzle inside of mushroom caps with olive oil, or add a sliver of cold butter.
Divide filling between mushroom caps.
Bake for 20 minutes or until tender.
Sprinkle with freshly grated Parmesan cheese. Serve immediately.