Slice the flank steak against the grain into thin strips (1/4-inch thick). In a bowl, whisk together cornstarch, soy sauce, brown sugar, ginger, and garlic. Set aside.

For extra tenderness, marinate the beef with soy sauce and a pinch of baking soda for 30 minutes (optional).

In a large pan, heat vegetable oil over medium-high until shimmering (2-3 minutes).

Add marinated beef in batches, stir-fry for 2-3 minutes until crispy, then remove and set aside.

Lower heat, add sauce mixture to pan, simmer for 1-2 minutes until slightly thickened.

Return crispy beef to the pan, stir to coat, heat through for another minute, and serve hot over rice with vegetables.