Creating Slow Cooker Mongolian Beef is easy when you follow these steps:

1. Prepare the Beef: Slice the flank steak against the grain into thin strips.
2. Mix Sauce Ingredients: In a bowl, combine soy sauce, brown sugar, minced garlic, minced ginger, and water. Stir until well blended.
3. Coat the Beef: Place the sliced beef in a large bowl. Sprinkle cornstarch over the beef, tossing to coat evenly.
4. Heat the Oil: In a skillet over medium-high heat, add vegetable oil. Once hot, sear the beef in batches until browned. This step is optional but adds flavor.
5. Transfer to Slow Cooker: Place the seared beef into the slow cooker. Pour the sauce mixture over the beef, ensuring it’s well coated.
6. Add Red Pepper Flakes: If you enjoy a bit of heat, sprinkle red pepper flakes over the beef.
7. Set the Slow Cooker: Cover and cook on low for 4-6 hours or high for 2-3 hours until the beef is tender.
8. Garnish and Serve: Once cooked, stir the beef and sauce. Serve over cooked rice or noodles, garnished with sliced green onions.

These simple instructions will guide you in preparing this incredible dish effortlessly.