Prep the steak: Slice the flank steak into 1/4-inch thin slices, across the grain. Lay the slices across a large baking sheet or your cutting board, and toss them on all sides with the 1/3 cup cornstarch.
Cook the steak: Heat a large pan or wok over medium-high heat. Once hot, add 1 to 2 tablespoons of oil. Cook the beef in small batches for about 30 seconds on each side, just until browned. Remove the beef to a plate and repeat with another batch, as needed.
Make the sauce: In a small bowl, whisk together the soy sauce, water, brown sugar, and cornstarch slurry. In the same large pan you used to cook the beef, turn the heat to medium, add the ginger and garlic, and sauté for 20 seconds. Add the sauce mixture and bring to a simmer. Cook, stirring, until slightly thickened, about 2 minutes.
Return beef to the pan with the sauce and toss to combine. Cook for 1 to 2 more minutes until warmed through. Add green onions.
Serve immediately, with a side of hot cooked rice.
