Cook bacon in a large skillet over medium heat until crispy; drain on paper towels. While the bacon is cooking, remove stems from mushrooms. Set caps aside, and chop stems.
Drain grease from skillet except for what clings to the pan. Add butter and olive oil to skillet with bacon drippings. When melted, stir in mushroom stems, onion, and garlic. Cook, stirring frequently until the onions caramelize, 15 to 20 minutes.
Preheat oven to 400 degrees F (175 degrees C). Spray a baking dish with cooking spray or olive oil
Once the onion mixture has reached a deep, rich brown color, place into a mixer bowl along with bacon, cream cheese, blue cheese, and bread crumbs. Mix on low until ingredients are combined. Brush the mushroom caps with olive oil. Stuff cheese mixture into mushroom caps and place into prepared baking dish.
Bake in preheated oven until bubbly and lightly browned, 10 to 15 minutes.
These can be assembled in advance and baked just prior to serving. Increased baking time may be needed.