Whisk together the soy sauce, oyster sauce, fish sauce, chili paste, and brown sugar in a small bowl.
Heat the vegetable oil in a wok or a large skillet over high heat. Add the garlic and chilies, if using, and cook until the garlic starts to brown, about 30 seconds.
Add the ground beef, using a spatula to press the beef flat across the pan. Cook undisturbed for about 2 minutes, or until browned. Flip over and cook for another minute. Break up the beef into smaller pieces. If there is a lot of excess fat in the pan, drain it off.
Stir in the onion and bell pepper and saute for about a minute. Add the sauce and stir everything together. Continue to cook for a few minutes, until the onions and pepper are tender and beef is cooked through. Sprinkle the Thai basil over top and stir until wilted.
Serve immediately over rice or topped with a fried egg, or both!
Notes
For more heat, add and saute Thai chilies slightly before the garlic in step 2.