Creating Slow Cooker Korean Beef is straightforward. Just follow these simple steps:

1. Prepare the Beef: Cut the beef chuck roast into 1-inch pieces. Trim any excess fat if necessary.
2. Mix the Sauce: In a bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and red pepper flakes (if using). Stir until the sugar is dissolved.
3. Place in Slow Cooker: Add the beef to the slow cooker and pour the sauce over it. Stir to ensure all pieces are well coated.
4. Cook: Cover the slow cooker and cook on low for 6-8 hours or high for 3-4 hours, until the beef is tender.
5. Thicken the Sauce: In the last 30 minutes of cooking, mix the cornstarch and water in a small bowl. Add this mixture to the slow cooker, stirring to combine. This will help thicken the sauce.
6. Finish Cooking: Cover and let it cook for another 30 minutes on high to thicken the sauce.
7. Garnish: Once cooked, serve hot, garnished with chopped green onions and sesame seeds.

These steps will guide you in creating this amazing dish with ease.