Preheat the oven to 400 degrees F and oil a pan with olive oil or cooking oil spray.
Remove the stems from the mushrooms. Set the caps aside and chop them finally.
Warm the olive oil in a medium sized pan, then add onions, garlic, chopped mushrooms, and green peppers. Saute for 5 minutes, until tender then remove from heat.
Add in 1/2 Panko breadcrumbs and parmesan into pan and stir everything together to combine.
Spoon the filling into the mushroom caps and place on prepared baking sheet. top with additional breadcrumbs and parm and drizzle a little olive oil on each one.
Bake in oven covered for 10 minutes, remove foil and bake additional 10 until golden brown and a little crispy on top. Serve immediately with favorite dipping sauce (i.e. marinara) or without.