Clean the mushrooms and gently remove the stems from them.
Place the mushrooms stem side down on a baking sheet and bake until the liquid leaks out of them then place the mushrooms stem side down on some paper towels to soak up any additional liquid.
Chop up about half the mushroom stems in very small pieces. Chop the onion, green onions, red bell pepper, roasted pepper and parsley.
Heat 2 tbsp of olive oil in a skillet. Add the mushrooms stems, red pepper, roasted pepper, green onion and onion to the skillet and saute. Add the oregano and salt and pepper. Saute for about 5 minutes until pepper and onion are tender.
Transfer the stuffing to a bowl and let it cool a bit. Add the parsley, breadcrumbs and Parmesan cheese and mix well.
Place the mushrooms stem side up on the baking sheet and fill them with the stuffing, generously.
Bake for 15 minutes.
Leftovers can be refrigerated in an airtight container for up to 4 days.
You can also make prepare these the day before and bake them right before serving.