Place the beef in the freezer 15 minutes before ready to begin, this will help firm it up for slicing.
Slice the beef against the grain in thin strips, about ¼ inch thick.
Add the cornstarch to a large ziploc bag or bowl. Place the meat in the bag/bowl and shake or stir, until well coated. Pour out into a strainer or colander and shake off the excess cornstarch.
Heat oil in a large deep skillet or wok over medium-high heat. Add the beef and quickly sty stir fry for 3-4 minutes, or until beef strips turn golden brown with crispy bits on the edges. Transfer beef to a plate.
In the same pan used to cook the beef strips, cook the garlic and ginger for 30 seconds over medium heat. Add the soy sauce, water, and brown sugar and stir to combine. Simmer for 2-3 minutes, or until the sauce starts to thicken up. Place the beef strips back in the pan, tossing the beef in the sauce.
Remove the pan from the heat. Garnish with green onions and sesame seeds and serve over white rice. Enjoy!