Preheat your oven to 375°F (190°C). Clean the large white mushrooms with a damp cloth to remove any dirt. Carefully remove the stems, setting them aside for later, leaving mushroom caps intact.
Finely chop the reserved mushroom stems, red bell pepper, onion, and green onions into small pieces.
Heat the olive oil in a large skillet over medium heat. Sauté the chopped onions until translucent, then add red bell pepper, chopped mushroom stems, and green onions. Cook for 5 minutes until softened. Stir in oregano, salt, and pepper.
In a mixing bowl, combine the sautéed vegetables with roasted red bell pepper, breadcrumbs, grated Parmesan cheese, and fresh chopped parsley. Mix well until combined.
Fill each mushroom cap generously with the vegetable mixture using a spoon.
Place the stuffed mushrooms on a baking sheet and bake for 20–25 minutes until golden brown and tender.
Remove from oven, let cool for a few minutes, and serve warm. Garnish with extra parsley if desired.