Gather your slow cooker. 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. Mix well until the brown sugar dissolves completely.

Cut your chuck beef into bite-sized cubes. Gently place the beef into the slow cooker, turning each piece to ensure it's well-coated with the flavorful sauce.

Set your slow cooker to low heat and let the mix simmer for about 8 hours.

About 30 minutes before finishing cooking, mix equal parts cornstarch and water until smooth for the thickening slurry.

Once your 8 hours are up, gently stir in the slurry mixture into the slow cooker with your beef. Switch to high heat and cover again.

Once thickened, spoon your Korean Beef into serving dishes. Garnish with sliced green onions and sesame seeds.