Place bacon in a large, deep skillet. Cook over medium-high heat until evenly browned. Drain, dice, and set aside.

Preheat oven to 400 degrees F (200 degrees C).

Remove mushroom stems. Set aside caps. Chop the stems.

In a large saucepan over medium heat, melt the butter. Slowly cook and stir the chopped stems and onion until the onion is soft. Remove from heat.

In a medium bowl, stir together the mushroom stem mixture, bacon, and 1/2 cup Cheddar. Mix well and scoop the mixture into the mushroom caps.

Bake in the preheated oven for 15 minutes, or until the cheese has melted.