01 - In a medium bowl, whisk together soy sauce, brown sugar, beef broth, sesame oil, rice vinegar, garlic, ginger, and gochujang until well combined.
02 - Place the beef cubes and sliced onion in the crock pot. Pour the sauce mixture over the beef and onions, tossing gently to coat evenly.
03 - Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the beef is fork-tender and easily shredded.
04 - Remove 1/4 cup of the cooking liquid and whisk it with the cornstarch until smooth. Stir the cornstarch slurry back into the crock pot, cover, and cook on high for another 20-30 minutes until the sauce thickens.
05 - Serve the Korean beef over steamed rice, noodles, or in lettuce wraps. Garnish with green onions and toasted sesame seeds.