Place beef strips in a bowl. Add all the ingredients for the marinade. Mix well until all the liquid is absorbed by the beef. Leave to combine for 10 minutes.
Mix all the ingredients for the sauce. Set aside.
Heat up a wok (or a deep frying pan) over a high heat then pour in 1½ tablespoons of oil. Sear the beef until it becomes pale. Dish out and set aside.
Discard any liquid in the wok. Pour in the remaining ½ tablespoon of oil. Add orange zest, fresh chilli, ginger and garlic. Cook until fragrant then stir in the beef.
Fry for 1 minute or so. Pour in the sauce (stir well beforehand). Dish out when beef is evenly coated with the thickened sauce. Serve immediately.