In a mixing bowl, combine the sliced flank steak with soy sauce, oyster sauce, fish sauce, and sugar. Let it marinate for about 15 minutes while you prepare the rest of the ingredients.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the minced garlic and sliced chilies, stir-frying for about 30 seconds until fragrant.
Add the marinated beef to the skillet, spread it in a single layer, and let it sear for about 1-2 minutes without stirring. This helps to develop a nice browning.
Stir-fry the beef for an additional 3-4 minutes until it is nearly cooked through, then add the red bell pepper. Continue to cook for another 2 minutes, ensuring everything is well combined.
Remove the skillet from the heat and fold in the fresh Thai basil leaves, allowing them to wilt from the residual heat.
Taste and adjust seasoning if necessary, adding more fish sauce or chilies if you like it spicier.
Serve the Spicy Thai Basil Beef over jasmine rice for a complete meal.
Notes
Adjust the number of chilies based on your spice preference.