Trim the excess fat from the chuck roast and cut it into 2-inch chunks for even cooking. In a mixing bowl, combine soy sauce, brown sugar, honey, sesame oil, rice vinegar, ginger, garlic, red pepper flakes, and black pepper. Add the sliced onion to the mix.

Place the seasoned chuck roast into your slow cooker and pour the prepared sauce over the beef. Set your slow cooker on low for 8-10 hours or high for 4-5 hours, until the beef is fork-tender.

Carefully remove the roast from the slow cooker and use two forks to shred the beef. If you prefer a thicker sauce, whisk together cornstarch and cold water to create a slurry, then stir it into the sauce and cook on high for an additional 15-20 minutes.

Return the shredded beef back to the slow cooker, mixing well to coat each bite with the sauce. Allow to simmer for an additional 10-15 minutes on low.

Serve over cooked rice, garnished with chopped green onions and sesame seeds. Optionally, serve with kimchi.