Preheat oven to 350 degrees.
Using a damp paper towel, wipe off mushrooms. (Do not clean mushrooms in water.) Cut around stem to pull out from base of mushroom. Cut stem into small dices. (We will use only 1/2 of the stems.)
Place mushroom caps in a greased or lightly buttered 9 x 13 baking dish.
In a skillet heat butter until it sizzles. Add diced mushroom stems, onions, and bell pepper. Stir for 3 minutes until softened. Add in bacon crumbles, black pepper, and cream cheese. Stir until all combined and cream cheese is melted in. Turn off burner.
Using a tablespoon or a 1 inch scoop, stuff each mushroom cap.
Mix together panko crumbs and parmesan cheese in a small bowl and sprinkle over the mixture on each mushroom cap.
Bake for 20-25 minutes until mushrooms are tender and topping is golden brown.
