Slice the flank steak against the grain into long strips. Put the strips into a medium-size mixing bowl.
Stir in soy sauce, hoisin sauce, sesame oil, brown sugar, garlic, ginger, and red pepper flakes until meat is evenly coated.
Heat a heavy skillet over high heat. Add in the olive oil and sear the meat until desired level of doneness is reached (2 to 3 minutes for medium rare).
Top with green onions (optional). Serve hot over cooked rice or rice noodles.
