Prepare the Sauce:
In a small bowl, combine the fish sauce, soy sauce, oyster sauce, and sugar. Stir until the sugar is dissolved and set aside.

Cook the Chicken:
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the ground chicken (or sliced chicken breast) and cook, breaking it up into small pieces, for about 5-7 minutes, or until the chicken is fully cooked and lightly browned.

Add the Aromatics:
Add the minced garlic and chopped Thai chilies to the skillet. Stir-fry for about 1 minute, until fragrant. If using onion and red bell pepper, add them now and cook for an additional 2-3 minutes, until they begin to soften.

Create the Sauce:
Pour the prepared sauce over the chicken mixture, adding water or chicken broth to help create a bit of sauce. Stir to coat the chicken and vegetables evenly in the sauce. Let it simmer for 1-2 minutes to allow the flavors to combine.

Add the Basil:
Stir in the fresh basil leaves and cook for 1-2 minutes, until they wilt and become fragrant.

Optional Fried Egg:
In a separate pan, heat a little oil over medium heat and fry eggs (one per serving) sunny-side up or to your desired doneness. The runny yolk adds an extra layer of richness when served over the chicken and rice.

Serve:
Serve the Thai Basil Chicken over steamed rice. Top with a fried egg (if desired) and extra fresh basil for garnish.