Mongolian Beef Noodles will quickly become a family favorite. It's easy to make, absolutely delicious, and leftovers are fantastic. If you have leftover beef/sauce, simply use it over rice or more noodles for tomorrow's dinner!
Heat 2 tsp oil in a saucepan over med-low heat, until hot but not smoking. Add ginger and garlic to the pan, stir for 30 seconds, and quickly add soy sauce and water. Add brown sugar and Sriracha in the sauce, stir well, and bring to boil at medium heat 2-3 min. Sauce should thicken a little. Remove from heat and set aside.
Slice flank steak against the grain into 1/4 inch thick bite-size slices. To get nice cuts, tilt blade of knife at 45 degree angle. Toss steak pieces into a large bowl with cornstarch. All pieces should be coated. Let beef sit 20 minutes (this step helps tenderize the beef.) Add 1/2 tsp table salt and toss to coat.
Meanwhile, heat 1/2 cup oil in a large wok or large skillet over medium until hot but not smoking. Add beef to pan and stir frequently, 2-3 minutes or until beef browns on the exterior. Beef will not be completely cooked, but transfer to a towel-lined plate. Pour oil out. Throw the meat back into the empty pan and stir over medium-low heat for 1 minute. Add the sauce, cook and stir for 1 minutes. Add green onions. Cook 1 minute, stirring, until beef is fully cooked through and onions are wilted. Keep warm.
In a large heavy pot of salted water, cook noodles according to package instructions. Drain but do not rinse. Toss hot noodles with desired amount of beef mixture. Garnish with additional Sriracha and chopped green onions.