In a medium-sized bowl, mix soy sauce, brown sugar, Mirin, garlic, sesame oil, and grated onion.
Add the beef short ribs to the mixed marinade in the bowl, covering the meat completely.
Cover the bowl with plastic wrap and place in the refrigerator for 1-6 hours.
Start your Traeger grill and turn it up to high. Make sure the grill is properly pre-heated.
Remove marinated meat from the refrigerator and place directly on the grill surface. Close grill as fast as you can, to make sure you lose as little heat as possible.
Cook for four minutes, then turn and cook on the other side for another four minutes. Pull the meat off the grill and serve on a bed of rice along with your favorite grilled vegetables.