Start by marinating the sliced flank steak. In a bowl, mix the soy sauce, oyster sauce, fish sauce, brown sugar, and lime juice. Add the beef slices, coating them well and let them marinate for at least 15-30 minutes.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Once hot, add the minced garlic and chopped bird's eye chilies, stirring quickly to avoid burning, for about 30 seconds until fragrant.
Increase the heat to high and add the marinated beef to the skillet. Stir-fry for about 3-5 minutes, or until the beef is browned and cooked through.
Toss in the sliced bell pepper and continue to stir-fry for another 2-3 minutes until the peppers are slightly tender but still crisp.
Remove from heat and add the fresh Thai basil leaves, stirring until they are wilted and mixed in thoroughly with the beef and vegetables.
Serve hot over cooked jasmine rice, ensuring everything is well combined.
Notes
Adjust the number of chilies based on your heat preference.