Cook bacon over medium high heat until crispy. Drain, chop and set aside. Preheat oven to 400 degrees F. Remove mushroom stems and finely chop. In a large saucepan over medium heat, melt the butter and slowly saute chopped stems and onion until the onion is soft. Remove from heat.

In a medium bowl, combine the mushroom stem mixture and bacon. Stuff mixture into the mushroom caps. Bake in the preheated oven approx 15 minutes. Top with cheese and heat until cheese is melted. Sprinkle with chives and serve warm.