Spread out the beef on a baking sheet and sprinkle the cassava flour over it. Toss to coat the beef evenly. Set aside.
Heat 1 tbsp oil in a skillet over medium-high heat. Add the carrots and cook for about 6-8 minutes stirring occasionally.
Add the green onions with the carrots and cook for another 5 minutes. Sprinkle with 1/4 tsp salt. Remove the carrots and onions from the pan. Set aside.
Clean out the pan with a wet cloth if needed. Return the pan to the stove and heat 1 tbsp of oil. Add half of the beef. Cook for about 2-3 minutes on one side (or until it starts to brown), then flip the pieces to the other side and cook for 2-3 minutes. Once browned, remove the beef from the pan and set it aside. Repeat the process with the second half of the beef.
In a small bowl, mix the sauce ingredients, then add them to the pan and heat the sauce over medium heat for 1-2 minutes or until it starts to bubble slightly.
Add the meat and vegetables back to the pan with the sauce and cook for 3-5 more minutes. This will warm the meat and veggies and allow the sauce to thicken.
Serve over cauliflower rice or Korean glass noodles. Garnish with green onions.
Notes
This dish is easiest to prepare in a non-stick pan. The coating on the beef just sticks very easily to a stainless steel pan. If you're not using a non-stick pan, be sure to use enough oil to prevent sticking.
Cassava flour seems to work best for coating the beef (it's much easier to work with) however if you don't have it, tapioca starch or arrowroot starch can be substituted.
Beef Cuts: While flank steak is usually used for Mongolian beef and works well, I found sirloin to be even more delicious because it is more tender. Other beef cuts that would work include skirt steak, thinly sliced beef tenderloin, short loin steaks, beef tenderloin, or New York strip.
The fish sauce is optional- If you'd like a slightly saltier sauce, add the fish sauce. For a sweeter sauce, omit it. I absolutely love it with the fish sauce and think it makes the dish. My family voted that they prefer it without the fish sauce because it is sweeter.
Storage: This will stay fresh in the fridge in a tightly sealed container for 2-3 days. Reheat it in the microwave or on the stove.
To Freeze: You can prepare the meal and freeze it in a tightly sealed container for at least a month. Thaw it in the fridge when you're ready to use it, then heat it on the stove.