Heat vegetable oil in a large skillet or wok over medium-high heat.
Add minced garlic and chopped chilies, sautéing for about 30 seconds until fragrant.
Increase the heat to high and add the thinly sliced beef. Stir-fry for 2-3 minutes until just browned but not fully cooked.
Add the sliced onion and bell pepper, continuing to stir-fry for another 2-3 minutes until the vegetables are tender-crisp and the beef is cooked through.
In a small bowl, mix together soy sauce, oyster sauce, fish sauce, and brown sugar. Pour the sauce over the stir-fry and toss everything to coat evenly.
Remove from heat and fold in the fresh Thai basil leaves until they are just wilted.
Serve immediately over cooked jasmine rice, garnished with lime wedges and chopped peanuts if desired.
Notes
Adjust the number of chilies to control the spice level.