Place flank steak slices in a shallow dish.

Mix soy sauce, sriracha sauce, green onion, sugar, whole dried peppers, garlic, sesame seeds, sesame oil, ground cayenne, and black pepper together in a bowl. Pour over steak. Cover with plastic wrap and refrigerate to marinate, at least 1 hour but preferably overnight.

Preheat an outdoor grill for high heat, and lightly oil the grate.

Remove steak slices from marinade and grill on preheated grill until slightly charred and cooked through, 1 to 2 minutes per side.

Simmer marinade in a wok or skillet over medium heat until thick and the consistency of gravy; pour over grilled steak.