Add the beef, brown sugar, soy sauce, onion, garlic, ginger, vinegar, sesame oil, and red pepper flakes to the insert of a slow cooker.
Cover and cook on low for 8 to 10 hours. The meat will be fork tender and fall apart. Use a fork to shred the beef and crush the garlic cloves. Stir and coat with the sauce.
Mix together the sour cream, lime zest, lime juice, and salt in a small bowl.
Heat the tortillas, top with beef, lime cream, kimchi, sriracha, cilantro, and sesame seeds.