Starting out with a cold skillet, cook bacon in a cast iron pan on medium high heat. Do not flip the bacon until it separates from the pan. The whole process should take approximately 10-15 minutes depending on the cut of bacon you choose. Place the cooked bacon on a plate lined with a paper towel. Once the bacon is cool, dice the bacon and set aside.
Add the onion and brussel sprouts to the hot cast iron skillet with bacon grease. Fry the diced onion and brussel sprouts on medium high heat for approximately 8-10 minutes.
Return diced bacon to the pan and season with salt and pepper.