1. Pat the beef dry and toss together in a bowl with the corn-starch and salt and black pepper to taste until completely coated. 2. Whisk together the orange juice, orange zest, sugar, soy sauce, ginger, garlic, rice wine vinegar and chilli flakes if using. Set aside.3. Heat the oil in a wok or large skillet. Once hot add the beef in small batches and cook until crispy, scooping out to drain on paper towelling and repeating until all of your beef has been cooked. Keep warm. 4. Add the pepper and onion to the remaining oil in the pan. Stir fry just until crispy tender. Add the orange juice mixture. Bring to the boil and thicken somewhat. Return the beef to the pan and heat through completely. 5. Serve hot spooned over white rice and garnished with the green onion.