Clean and remove stems from mushrooms. Mince the stems and put in a bowl; set aside.
Cook bacon in a skillet until crispy. Transfer bacon to paper towels, leaving bacon fat in the skillet. Add butter, mushroom stems, onions and garlic. Cook until onions caramelize.
In a food processor, add the bacon, blue cheese, cream cheese, the mixture from the skillet, and bread crumbs. Pulse in food processor a couple times until everything is incorporated. Do not blend.
Fill mushroom caps with the cheese mixture and place them in Basket.
Set to 350°F for 15 minutes. Cook until bubbling and tops are light brown. Serve hot.
