In a large mixing bowl, whisk together soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and black pepper.

Add the thinly sliced flank steak to the marinade, ensuring all pieces are well-coated. Let it marinate for at least 15 minutes.

Place your skillet over medium-high heat and allow it to get hot.

Add the marinated beef in a single layer to the skillet. Avoid overcrowding; cook in batches if necessary.

Cook the beef for about 2-3 minutes without stirring, allowing it to develop a nice brown crust.

Once the underside is browned, stir the beef and cook for another 2-3 minutes until cooked through.

Remove the skillet from heat and sprinkle chopped green onions and sesame seeds on top.

Serve the beef over steamed rice, noodles, or in lettuce wraps.