Heat the vegetable oil in a large skillet or wok over high heat until shimmering.
Add the minced garlic to the oil and sauté for about 30 seconds until fragrant, being careful not to burn it.
Add the sliced beef to the skillet, spreading it out in a single layer. Let it sear for about 2 minutes without stirring, then stir-fry for another 2-3 minutes until browned and cooked through.
Once the beef is cooked, add the sliced red bell pepper and green beans to the skillet. Stir-fry for an additional 2-3 minutes until the vegetables are just tender but still crisp.
In a small bowl, mix together the soy sauce, oyster sauce, fish sauce, and brown sugar. Pour this sauce over the beef and vegetables, stirring well to coat everything evenly.
Add the chili flakes for a kick and stir in the fresh Thai basil leaves, allowing them to wilt slightly in the heat. Stir everything for another minute until combined.
Remove from heat and serve hot over a bed of jasmine rice.
Notes
Serve the dish in a large bowl, garnished with extra basil leaves on top and a sprinkle of chili flakes for added color.