Combine brown sugar, ginger, garlic, green onion, chili flakes, mirin, sesame oil, and soy sauce in a gallon Ziploc bag. Close bag and shake well to combine.
Cut off any excess fat from the short ribs and add to the bag. Shake to combine.
Tightly seal the bag and marinate the short ribs in the refrigerator for at least 3 hours and up to 24 hours (longer is better).
Pour short ribs and marinade into a Dutch oven. Cover and bake at 250 degrees for 3 1/2 – 4 hours.
Transfer the beef to a bowl and shred with a fork.
Serve on corn tortillas topped with Korean slaw and optional cilantro and/or pickled red onions.
