Gather all ingredients.
Whisk soy sauce, green onion, sugar, garlic, sesame seeds, sesame oil, and pepper together in a bowl.
Place flank steak slices in a shallow dish. Pour marinade over top. Cover and refrigerate for at least 1 hour or overnight.
Preheat an outdoor grill for high heat, and lightly oil the grate.
Quickly grill flank steak slices on the preheated grill until slightly charred and cooked through, 1 to 2 minutes per side.
Serve hot and enjoy!
