In a small saucepan, bring onion, 2 tablespoons vinegar, sugar, and red pepper to a boil over medium heat. Cook, stirring frequently, until onions are soft and liquid has thickened, about 4 minutes. Remove from heat, and let cool completely.
In a large bowl, whisk together honey, mustard, salt, black pepper, and remaining 1 tablespoon vinegar until well combined. Stir in sprouts, apple, and onion mixture until well combined. Serve immediately.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/honey-mustard-brussels-sprouts-slaw-recipe/