Preheat your oven to 300°F (150°C).
In a large bowl, whisk together the soy sauce, brown sugar, honey, apple cider vinegar, minced garlic, ground ginger, black pepper, and red pepper flakes until well combined.
Place the beef flanken ribs in a large baking dish and pour the marinade over the ribs, ensuring they are well coated. Cover the dish with aluminum foil.
Bake in the preheated oven for 2.5 to 3 hours, or until the ribs are tender and the meat easily pulls away from the bone.
Remove the foil during the last 30 minutes of cooking to allow the ribs to caramelize and develop a nice glaze.
Once done, remove from the oven and let the ribs rest for about 10 minutes. Drizzle with sesame oil and garnish with chopped green onions before serving.