In a small bowl, whisk together the oyster sauce, soy sauce, fish sauce, dark soy sauce, and brown sugar until the sugar has dissolved.

Heat 1 tablespoon of vegetable oil in a wok or large skillet over high heat. Once smoking hot, add the sliced beef in a single layer.

Sear the beef for 1-2 minutes without moving it to get a nice char, then stir-fry for another minute until browned but not fully cooked through; remove from the pan and set aside.

Add the remaining tablespoon of oil to the same pan, then toss in the minced garlic and sliced chilies. Stir-fry for about 30 seconds until fragrant, being careful not to burn the garlic.

Add the red bell pepper and green beans to the pan. Stir-fry vigorously for 2-3 minutes until the vegetables are tender-crisp and vibrant.

Return the cooked beef and any accumulated juices to the pan, then pour the prepared sauce over the mixture.

Toss everything together for 1-2 minutes, ensuring the meat and vegetables are evenly coated in the glossy sauce.

Turn off the heat and immediately fold in the Thai basil leaves. Stir just until the basil is wilted and aromatic, then serve immediately over hot jasmine rice.