Combine diced onion, crushed garlic, minced ginger, rice wine vinegar, sesame oil, soy sauce, beef stock, brown sugar, and red chili flakes in the slow cooker.

Cut the chuck beef into bite-sized cubes and add to the sauce mixture.

Cover and cook on low for about 8 hours.

Whisk cornstarch and water together in a bowl until smooth.

Stir the cornstarch mixture into the slow cooker near the end of cooking.

Increase heat to high and cook for an additional 30 minutes.

Serve garnished with green onions and sesame seeds.