In a liquid measuring cup, add all the ingredients for the stir fry sauce. Slice the scallions and set aside. Allow the ginger, garlic, and Szechuan peppercorns to soak while you slice the beef.
Lay the flat iron steak on a cutting board. Cut the beef against the grain into thin slivers. Then toss the beef strips with the corn starch.
Set a wok (or XL skillet) over high heat. Add both oils. When the oil starts to smoke, carefully toss in the beef strips and Szechuan chiles. Quickly move them around the wok to stir fry the beef on both sides.
Once the beef is mostly cooked (2-4 minutes) stir in the sauce. Allow the sauce to simmer and thicken. Once it reaches your desired consistency, turn off the heat. Serve warm, sprinkled with chopped scallions and sesame seeds.
You can find Szechuan peppercorns and chile peppers at a grocery stores with good international sections, at Asian markets, and online.
Although Szechuan chile peppers are traditionally served in the stir fry, they are not meant to eat. You can take them out before serving if you like.
If you are very sensitive to heat, skip the Szechuan chile peppers and use a small amount of crushed red pepper instead.
Shaoxing wine is Chinese, yet Mirin is Japanese and often much easier to find. You can use either.