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

1. Prepare the Beef: Thinly slice the flank steak against the grain into bite-sized pieces.
2. Mix the Sauce: In a bowl, combine soy sauce, brown sugar, minced garlic, and minced ginger. Stir well until the sugar dissolves.
3. Place in Slow Cooker: Add the sliced beef to the slow cooker.
4. Add Sauce: Pour the sauce mixture over the beef, ensuring all pieces are well-coated.
5. Cook: Cover the slow cooker and cook on low for 5 to 6 hours or on high for 3 to 4 hours, until the beef is tender.
6. Thicken the Sauce: About 30 minutes before serving, mix cornstarch and water in a small bowl until smooth. Stir into the slow cooker to thicken the sauce.
7. Add Heat (Optional): If you like a bit of spice, stir in red pepper flakes at this point.
8. Serve: Once the sauce has thickened, serve the Mongolian beef over cooked rice or noodles.
9. Garnish: Top with chopped green onions for added flavor and a pop of color.

These simple steps will guide you through making a dish that is flavorful and satisfying.