Prepare the beef roast by trimming excess fat and cutting into large chunks.
In a bowl, mix together soy sauce, brown sugar, rice vinegar, minced garlic, ginger, and sesame oil.
Place the beef chunks in the slow cooker and pour the marinade over them.
Cover the slow cooker and set it to low for 8 hours or high for 4 hours.
Towards the end of cooking, check the sauce consistency and add cornstarch if preferred.
Once cooked, shred the beef with two forks and stir it back into the sauce.
Serve hot, garnished with chopped green onions.