Add chuck roast and the quartered onion to slow cooker.
In a small bowl, whisk together the garlic, crushed red pepper flakes, black pepper, ginger, brown sugar, soy sauce, broth, vinegar and sesame oil; pour over beef and onion.
Cover slow cooker and cook on LOW for 8 hours.
After 8 hours, using two forks, shred the beef in the slow cooker.
Serve shredded beef on tortillas, top with Korean BBQ sauce, kimchi, Sriracha mayo, cucumber, red onion, cilantro and sesame seeds.