Creating Slow Cooker Korean Beef is simple and requires just a few straightforward steps:

1. Prepare the Beef: Slice the flank steak against the grain into thin strips for maximum tenderness.
2. Mix the Marinade: In a medium bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, ginger, black pepper, and red pepper flakes if using. Stir until the sugar is dissolved.
3. Combine in Slow Cooker: Place the sliced beef in the slow cooker. Pour the marinade over the beef, ensuring all pieces are well-coated.
4. Cook: Cover the slow cooker and set it to cook on low for 8 hours or on high for 4 hours. The beef should be tender and infused with flavor.
5. Garnish: Once cooked, stir the beef gently. Serve hot, garnished with sliced green onions and sesame seeds.
6. Serve: Spoon the Korean beef over bowls of cooked rice for a complete meal.

These steps will guide you in creating a restaurant-quality dish right in your home.