Wrap steak in plastic wrap, and place in the freezer for 30 minutes. Unwrap and slice across the grain into 1/4-inch thick slices.
In a large bowl, combine flank steak and cornstarch.
Heat vegetable oil in a large cast iron skillet. Working in batches, add beef and fry until browned and cooked through, about 1-2 minutes. Transfer to a paper towel-lined plate; discard excess oil.
Add beef and soy sauce mixture to the skillet over medium heat and cook, stirring occasionally, until sauce thickens, about 2-3 minutes. Stir in green onions.
Serve immediately, garnished with sesame seeds, if desired.
for the soy sauce mixture
In a small saucepan over medium heat, combine water, brown sugar, soy sauce, garlic, ginger and vegetable oil. Bring to a boil; reduce heat and simmer, stirring occasionally, until slightly thickened, about 5-10 minutes.
Notes
Freeze the steak for slicing. Throwing the steak in the freezer for 20-30 minutes will make it easier to slice the meat into very thin, even pieces.
Slice against the grain. Cut the steak against the grain, slicing perpendicular to the direction of the muscle fibers. This shortens the fibers so that the meat becomes incredibly tender rather than chewy.
Cornstarch is key. The cornstarch is a key ingredient in two ways. It helps to tenderize the beef, keeping it tender and juicy while developing that favorited light, crispy crust. It also serves as a thickening agent, creating that glossy sauce that coats the steak oh-so-perfectly.
Use reduced sodium soy sauce.Reduced sodium has less sodium and less salt without compromising flavor.
Use a cast iron skillet. A heavy bottom pan, such as a cast iron skillet, retains heat much better than a nonstick skillet, allowing for even cooking, a better sear and smoky, caramelized flavors.
Cook in batches. Work in batches to properly sear the meat for maximum caramelization and smoky flavors. An overcrowded pan will unfortunately steam the beef instead.
Mix it up. Add broccoli, asparagus or green beans. The added vegetables will soak up all the sauce, perfect for those picky eaters and toddlers.
Freeze as needed. Mongolian beef is a dish that freezes very well, always freezing the beef separately from the rice to preserve the texture and prevent sogginess.