Combine onions, garlic, apple cider vinegar, soy sauce, brown sugar, and bay leaf in slow cooker. Season with black pepper. Place chicken thighs atop mixture. Sprinkle paprika over chicken thighs.
Cover and cook on Low for 8 hours.
Switch slow cooker to High. Add bok choy to chicken mixture; cook another 5 minutes. Garnish with green onion.
