Cook the bacon until crisp and brown in a large skillet over medium heat, about 10 minutes; transfer bacon to a bowl, but leave the grease in the skillet. Cook the onions in the bacon grease until browned, about 10 minutes, stirring frequently. Reduce heat to low, and cook the onions until very soft, brown, and sweet, another 10 to 15 minutes. Stir often. Mix the onions with the bacon in the bowl, and set aside.