Trim ends of Brussels sprouts, and finely slice. Rinse Brussels sprouts in a colander, and pat dry.
In a large nonstick skillet, cook bacon over medium heat for 4 to 6 minutes or until crispy. Place bacon on paper towels to drain. Crumble bacon. Drain grease from pan, reserving 3 tablespoons.
Core apple, and slice into bite-size pieces.
In a small bowl, whisk together brown sugar, vinegar, salt, and pepper.
Heat reserved bacon grease in skillet over medium-high heat. Add Brussels sprouts, and cook for 3 to 4 minutes or until they begin to brown slightly, stirring often. Add apple to skillet, stirring to combine; cook for 1 minute. Pour brown-sugar mixture over sprouts mixture. Cook for 1 minute or until sugar has dissolved, stirring often. Add bacon, stirring to combine.
