Prepare the Beef: Slice the flank steak thinly against the grain to ensure tenderness. Let it rest at room temperature for about 15 minutes.

Brown the Beef: Sear the beef in a skillet over medium-high heat until browned, doing this in batches.

Combine the Sauce: In a bowl, mix together soy sauce, brown sugar, beef broth, minced garlic, and grated ginger.

Slow Cook: Pour the sauce over the browned beef in your slow cooker. Cook on low for 4-5 hours or high for 2-3 hours.

Thicken the Sauce: Create a slurry by mixing cornstarch with water. Stir this into the slow cooker and cook on high for an additional 30 minutes.

Serve: Garnish the beef with sliced green onions and sesame seeds and serve it over rice or noodles.