In a large bowl, mix the cornstarch and water together. Then add the ground ginger, soy sauce, beef stock, and dark brown sugar.
Dump marinade off the beef if it is not all soaked in then add ¼ cup cornstarch to the bowl, and with your hands, lightly coat the beef. (See Note)
Heat ¼ cup oil in a wok or small skillet over medium-high heat.
Once the oil is hot, add ½ the beef and cook for around 45 seconds or until crisp.
Flip beef, and cook for another 30-45 seconds, then remove and put on a paper towel.
Repeat with the other half the beef. This helps it get crisp instead of steaming the meat, see note.
Remove all but about a tsp of oil from the wok, and add ginger and garlic to the pan. Saute, but don't burn. Should only take 15-20 seconds.
Then add in the sauce.
Bring it to a simmer and let it cook down to a thick, sticky, sauce. (About 1-2 minutes)
Add the beef and toss.
Cut green onions, and toss in, cook for 30 more seconds.
Serve over rice immediately!