"The secret ingredient in this dish is Chinese black vinegar. If you don't have this, substitute with young, unaged balsamic vinegar. The aged, good quality balsamic vinegar is too sweet - so make sure you get the cheaper, young balsamic. To keep this dish vegetarian, replace the beef with fresh, thick, meaty shitake mushrooms (cut in half) or even sliced portobello mushrooms. TO CUT FLANK STEAK ACROSS THE GRAIN: in the middle of this post for Flank Steak with Goat Cheese Recipe, I have a couple of photos showing you exactly how to cut flank steak across the grain. This is an important step because flank and skirt steak both are very tough cuts -- however -- if you cut it correctly, the steak becomes incredibly tender. Make sure you don't skip this step!..."