Pat the flank steak dry with paper towels. Slice the beef thinly, about 1/4-inch thick, against the grain. This ensures tenderness.

In a medium bowl, combine the sliced beef with baking soda and 2 tablespoons of soy sauce. Mix thoroughly to coat all the beef. Let it sit at room temperature for 15 minutes; the baking soda is a tenderizer.

After marinating, rinse the beef under cold running water to remove the baking soda, then pat it *very* dry with paper towels. This step is crucial for achieving a crispy crust.

Toss the dried beef with 1/4 cup of cornstarch until each piece is evenly coated. Set aside.

In a small bowl, whisk together 1/2 cup soy sauce, dark brown sugar, hoisin sauce, rice vinegar, water or beef broth, toasted sesame oil, and 1 tablespoon of cornstarch until smooth. Set aside.

Heat 1/2 cup vegetable oil in a large wok or skillet over medium-high heat until shimmering (around 350-375°F). Add the coated beef in a single layer, frying in batches if necessary to avoid overcrowding the pan. Fry for 2-3 minutes per side until golden brown and crispy. Remove the cooked beef with a slotted spoon to a paper towel-lined plate to drain excess oil. Discard most of the oil from the pan, leaving about 1 tablespoon.

Reduce the heat to medium. Add 1 tablespoon of vegetable oil (if needed), then add the grated ginger and minced garlic to the pan. Sauté for 30 seconds until fragrant. Add the white and light green parts of the chopped green onions and cook for another minute.

Give the prepared sauce a quick whisk (as cornstarch can settle) and pour it into the pan. Bring the sauce to a simmer, stirring constantly, until it thickens to your desired consistency, about 1-2 minutes.

Return the cooked beef to the pan. Add the dark green parts of the green onions. Toss everything together until the beef is well coated in the savory sauce and heated through.

Serve immediately over steamed white rice.