Soak 3-4 black mushrooms in water for 10 minutes. Slice thin. With flank steak, slice and rub with a little bit, maybe 1/2 tsp of corn starch and let it sit. Then heat the oil, add the mushrooms, soy sauce, corn syrup/sugar, black beans then the meat. Near the end, add the green onions and season with salt. If you want an extra kick of flavor, you can add a little bit of rice vinegar too.