Add chopped bacon, onions and halved brussel sprouts to a cast iron skillet over medium heat. Season with salt and pepper and cook, stirring occasionally, until bacon is cooked and brussel sprouts have begun to brown, about 15 to 20 minutes.
Drizzle 1/2 cup of the balsamic vinegar over bacon and brussels, stir and cook a minute more.
Take off a heat. Transfer to serving platter and drizzle remaining balsamic vinegar over top. Enjoy.