Prepare the Slow Cooker: Add the cubed 2 1/2 to 3 pounds chuck roast to a 5-quart slow cooker.
Make the Sauce: In a medium mixing bowl, whisk together the 1/2 cup beef stock, 1/2 cup low sodium soy sauce, 3/4 cup packed light brown sugar, 1 tablespoon Sriracha or chili paste, 1 tablespoon rice wine vinegar, 5 cloves garlic, 2 teaspoons minced fresh ginger, 1 tablespoon sesame oil, and 1/4 teaspoon red pepper flakes (if using). Pour the sauce over the beef.
Slow Cook: Cover the slow cooker with the lid and cook on low for 7 to 8 hours, or on high for 3 to 4 hours, until the beef is fork-tender.
Thicken the Sauce: During the last 30 minutes of cook time, whisk together the 2 tablespoons cornstarch and 3 tablespoons water in a small bowl to form a slurry. Pour the slurry into the slow cooker and stir gently to combine. Cover and continue cooking on high for 30 minutes or until the sauce is thickened.
Serve and Garnish: Stir the beef gently in the thickened sauce, then serve hot over steamed rice or noodles. Garnish with chopped green onions and a sprinkle of sesame seeds.
Notes
Make-Ahead Tips: You can prep all the ingredients ahead of time and store the beef and sauce separately in the refrigerator for up to 24 hours. When ready to cook, pour everything into the slow cooker and follow the instructions.Storage: Store leftovers in an airtight container in the refrigerator for up to 3 days.Reheating Tips: Reheat individual portions in the microwave in 30-second intervals, stirring in between, until heated through. For stovetop reheating, warm gently in a saucepan over medium-low heat.Freezing Tips: Freeze fully cooked and cooled Korean BBQ beef in airtight containers or resealable freezer bags for up to 3 months. Thaw overnight in the refrigerator and reheat as directed.