In a large mixing bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, gochujang, and black pepper. Mix well to create a marinade.
Add the cubed beef chuck roast to the marinade and toss to coat all pieces. Let it marinate for at least 30 minutes, or overnight in the refrigerator for deeper flavor.
Once marinated, transfer the beef and all the marinade into the slow cooker. Add the sliced onion on top.
Cover the slow cooker and set it to low heat. Cook for 6-8 hours, or until the beef is tender and easily shredded.
Once cooked, shred the beef in the slow cooker using two forks and stir to combine with the sauce and onions.
Serve the Korean beef sides over cooked jasmine rice. Garnish with chopped green onions and a sprinkle of sesame seeds for added flavor and crunch.