Remove the bottom part of the mushroom. Dice the bottoms along with the garlic, red onion, and red bell pepper and saute in pan with a little olive oil, salt, and pepper. Then add the mixture to a bowl and mix with the Panko bread crumbs. The bread crumbs will soak up the moisture from the garlic, onion, and pepper mixture. With a spoon scoop the mixture into the mushroom caps. Place the filled caps on a cookie sheet and spray the tops of them with a little olive oil. Bake at 350 degrees for 15-20 minutes until golden brown and crispy on top.