Start off by making the sauce: Mince 1 garlic close and 1 tbsp ginger and cook it in a saucepan filled with 2 tbsp vegetable oil. Cook it over medium-high heat for 1 minute.
Add the 1/4 cup brown sugar, 1 cup low sodium soy sauce, and 1/2 cup water. Bring to a simmer and cook over medium-low heat for 5 minutes.
Thinly slice the 2 lb. beef into small pieces. Pro tip: place the beef in the freezer 15-20 minutes prior to slicing to make thinner cuts without the meat sliding around.
Place the sliced beef in a large bowl and mix it with 1/3 cup cornstarch.
Heat 1/3 cup vegetable oil in a 14-inch wok and cook the beef in 2 batches to avoid overcrowding and mushy texture. Once cooked, remove the beef and set it aside.
In the same wok, cook 1 clove minced garlic over medium heat for 1 minute. Then add 1 cup sliced yellow onion and cook until translucent.
Add the beef back into the wok and pour in the sauce. Bring to a simmer for another 5 minutes.
Add 2 bunches of sliced green onion to the beef and stir for about 2-3 minutes. Serve warm over hot rice.
Slicing the meat- A great way to get thinly sliced beef is by having the meat partially frozen when you cut it. That way it doesn't slide around and you achieve super thin slices.
Cook in batches- In order to get nice and crispy pieces of cooked beef, don't overcrowd the wok. Cook the beef in batches. Overcrowding will cause a lot of liquid to collect and result in mushy grey looking beef.