Preheat oven to 375 degrees F. Heat a cast-iron or nonstick ovenproof skillet, and cook bacon according to package directions; remove and drain on paper towels. Reserve a tbsp. or two of the grease, and heat on medium-high. When hot, add the onions and mushrooms, stirring frequently until the onions are soft and the mushrooms release then reabsorb their liquid, about 5-10 minutes.