Wipe the mushrooms with a barely damp paper towel. DO NOT WASH MUSHROOMS!!
Remove the stems from mushrooms and scrape out the centers with either a melon baller or a measuring teaspoon.
Mix the two cheeses together. Set 1/4 of the cheese aside.
Saute chopped bacon with chopped onion 5-7 minutes on medium high heat until bacon is cooked through and browned. Stir frequently so you don't burn your onions. Transfer the bacon and onion mix to a bowl.
Once the bacon is cooled to room temperature, mix it into the larger portion of cheese.
Place a heaping tablespoon into each mushroom so it's plump and full, than top each mushroom with with extra cheese.
Bake at 400˚F for 8 minutes or until they are soft to the squeeze and just starting to look a little juicy. Don't overcook them or they will get soggy.