Start by slicing your beef as thinly as possible against the grain. Pro tip: Placing the beef in the freezer for 15-20 minutes before slicing makes it firmer and easier to cut into paper-thin pieces.
While the beef is chilling, slice your vegetables and mince the garlic. Having everything prepped before you begin cooking is essential as this dish comes together quickly.
In a small bowl, whisk together the oyster sauce, soy sauce, fish sauce, brown sugar, water, and cornstarch until smooth and the sugar has dissolved.
Heat 1 tablespoon of oil in a large wok or skillet over high heat until it's just beginning to smoke. Add half the beef in a single layer and let it sear undisturbed for 1 minute before stirring.
Cook for another minute until the beef is browned but still slightly pink in the center. Transfer to a plate and repeat with remaining beef.
Return the wok to high heat and add the remaining tablespoon of oil. Add the garlic and Thai chilies, stirring constantly for 30 seconds until fragrant but not browned.
Immediately add the sliced bell pepper and onion, stir-frying for 2-3 minutes until they begin to soften but still retain some crispness.
Return the beef to the wok, along with any accumulated juices. Give the sauce mixture another quick whisk and pour it into the wok.
Stir-fry for 1-2 minutes until the sauce thickens and coats everything with its glossy sheen.
Turn off the heat and add the Thai basil leaves, tossing quickly to wilt them just slightly.
Serve the beef mixture over jasmine rice, topped with a fried egg if desired. Garnish with sliced scallions, lime wedges, and cucumber slices.