Cut beef steak into thin strips by cutting across the grain on meat. Transfer to a mixing bowl and pat dry with a kitchen towel, if moist.
Add cornflour, salt and pepper, mix well.
Heat 1 tablespoon oil in a non-stick pan or wok. Add ⅓ of beef strips and fry for 1-2 minutes on medium heat until golden.
Add another 1-2 tablespoon of oil and fry another batch of the beef strip. (If the pan has any burnt bits, wipe it with a paper towel before introducing the second batch and add 1 tablespoon oil or as needed to fry.)
Similarly, fry all the strip in 3-4 batches without crowding the pan, set aside.
Heat 1 tablespoon oil in the same pan. Fry ginger, garlic for 1 minute until sizzling hot.
Add chicken cube, soy sauce, chili garlic sauce, brown sugar, vinegar, and red chili flakes, mix well.
Stir in sliced onion, capsicum and green chilies, fry for 2 minutes until slightly soft.
Add the fried beef and mix well. (If your beef is under-cooked, you can cook for 2-3 minutes by covering the pot on medium heat.)
Make cornflour slurry by mixing cornflour and water. Add to the wok. Cook for 1 minute until sauce thickens and shines.
Just before serving toss garnish with sliced green onions (green parts only) and serve.