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1. Start by slicing the beef into thin strips, about 1/4 inch thick. This helps the beef cook quickly and evenly. If you find it difficult to slice, you can freeze the beef for about 30 minutes to firm it up, making it easier to cut.
2. In a large bowl, combine the cornflour, baking powder, salt, and pepper. This mixture will create a crispy coating for the beef.
3. Add the beef strips to the bowl and toss them in the cornflour mixture until they are well coated. Make sure each piece is evenly covered for that perfect crunch.
4. In a large frying pan or wok, heat about 1-2 inches of oil over medium-high heat. You want the oil to be hot enough that a small piece of beef sizzles when added.
5. Once the oil is hot, carefully add the coated beef strips in batches. Avoid overcrowding the pan, as this can lower the oil temperature and result in soggy beef.
6. Fry the beef for about 3-4 minutes or until golden brown and crispy. Use a slotted spoon to remove the beef from the oil and place it on a plate lined with paper towels to drain excess oil.
7. Repeat the frying process with the remaining beef strips, ensuring the oil temperature remains consistent.
8. In a small bowl, mix together the soy sauce, oyster sauce, rice vinegar, and sugar. Stir until the sugar is dissolved. This will be the base of your sauce, giving it a sweet and savory flavor.
9. In the same pan used for frying (drain excess oil if necessary), add a tablespoon of oil if needed and heat over medium heat.
10. Add the minced garlic and ginger to the pan, sautéing for about 30 seconds until fragrant. Be careful not to burn them!
11. Next, add the sliced onion, bell pepper, and red chillies. Stir-fry for about 2-3 minutes until the vegetables are slightly softened but still crisp.
12. Pour the prepared sauce over the vegetables in the pan, stirring to combine. Let it simmer for about 1-2 minutes, allowing the flavors to meld together.
13. Once the sauce has thickened slightly, add the crispy beef strips back into the pan. Toss everything together gently, ensuring the beef is well coated with the sauce.

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