Wash mushrooms and prepare a small baking dish coated with cooking spray.
Remove stems from the mushrooms and set aside. Place mushroom caps in baking dish.
Chop bacon into chunks and cook until browned; drain in paper towel lined bowl and set aside.
Now chop up mushroom stems and onion and saute with butter over medium heat until softened. Turn heat to low and add in cream cheese, bacon, parmesan cheese and 1/4 cup of the cheddar cheese. Stir until cheese is all melted and incorporated. Spoon the mixture into the mushroom caps.
Top with remaining cheddar cheese and place in preheated oven at 400 degrees. Bake 10 to 15 minutes or until hot and bubbly. Enjoy!
