Place the beef & 1/4 cup cornstarch in a resealable bag. Shake to coat evenly.
Heat the vegetable oil in a pan over high heat.
Add the meat in a single layer & season to taste w/ salt & pepper (keep in the mind that the sauce has plenty of salt in it). Cook for 3-4 minutes per side or until browned. Cook in multiple batches if needed.
Remove the meat from the pan & place on a cooling rack to drain excess oil. Cover with parchment paper to retain its warmth.
Add the garlic & ginger to the pan & cook for 30 seconds. Add the soy sauce, sesame oil, water & brown sugar to the pan & bring to a simmer.
Mix a teaspoon of cornstarch w/ 1 tablespoon of cold water. Add the cornstarch to the sauce & bring to a boil for 30-60 seconds until just thickened.
Add the meat & green onions to the pan & toss to coat w/ the sauce. Serve over rice or any side dish of your choice.