Prepare the sauce

• In a bowl, whisk together the beef stock, soy sauce, dark soy sauce, brown sugar and cornstarch until smooth.

Sear the beef

• Heat the oil in a frying pan over high heat.
• Add the sliced flank steak in a single layer and sear until browned, then set aside.

Cook the aromatics

• Add the onion and garlic to the pan and stir-fry until fragrant and softened.

Simmer the sauce

• Pour the sauce into the pan and let it bubble and thicken.

Combine and serve

• Return the beef to the pan and toss to coat in the sauce.
• Add the green onions, toss briefly and serve hot.