Prepare the Beef:

Toss the sliced beef in cornstarch, ensuring each piece is fully coated. Shake off any excess cornstarch.

Fry the Beef:

Heat the oil in a large pan or wok over medium-high heat. Add the beef in batches, frying until crispy and browned, but not fully cooked through. Remove the beef and set it aside.

Make the Sauce:

Discard the excess oil, leaving about 1 tablespoon in the pan. Add ginger and garlic, and sauté for a few seconds until fragrant.
Add soy sauce, water, and brown sugar to the pan. Cook the sauce until it thickens slightly.

Make the Sauce:

Discard the excess oil, leaving about 1 tablespoon in the pan. Add ginger and garlic, and sauté for a few seconds until fragrant.
Add soy sauce, water, and brown sugar to the pan. Cook the sauce until it thickens slightly.

Finish the Dish:

Stir in the green onions (and red pepper flakes or dried chili if using) and cook for an additional minute.

Serve:

Serve the Mongolian Beef over steamed rice or noodles.