Start by slicing the flank steak into thin strips against the grain. This will ensure the beef is tender and easy to chew. If you find it challenging to cut, placing the steak in the freezer for 30 minutes can make it easier to slice.
In a mixing bowl, combine the soy sauce, water, minced ginger, and minced garlic. Add the sliced beef to the marinade and let it sit for at least 15 minutes to absorb the flavors.
After marinating, drain the beef and discard the marinade. In another bowl, coat the beef strips with cornstarch evenly. This will create a crispy outer layer when fried.
In a large skillet or wok, heat 1 cup of vegetable oil over medium-high heat. Once the oil is hot, carefully add the beef strips in batches, making sure not to overcrowd the pan.
Fry the beef for about 2-3 minutes on each side until golden brown and crispy. Use a slotted spoon to remove the beef and place it on a paper towel-lined plate to drain excess oil.
In the same skillet, remove excess oil, leaving about 1 tablespoon. Add the dark brown sugar and stir until it begins to dissolve. Then, add the remaining marinade (soy sauce mixture) and bring it to a simmer.
Once the sauce is slightly thickened, return the fried beef to the skillet. Toss to coat the beef in the sauce. If you like a little heat, sprinkle in the red pepper flakes at this stage. Finally, add the green onions and stir-fry for an additional minute before serving.
