Heat your large heavy-bottomed pot over medium heat. Add a splash of oil and let it warm until shimmering.
Add sliced onion, minced garlic, and ginger to the pot. Sauté for 2-3 minutes until fragrant, with the onion turning translucent and the mixture bubbling gently.
Stir in the chicken pieces and cook for about 5 minutes, stirring occasionally, until the chicken turns white on the outside and starts to brown slightly.
Pour in the chicken broth and add the fish sauce. Bring the mixture to a gentle simmer and cook for 10 minutes, allowing the flavors to meld and the chicken to become tender.
Add the coconut milk (if using) and sliced Thai chili to the pot. Stir well and cook for another 2 minutes, letting the broth warm through.
Remove the pot from heat and stir in lime juice. Taste the broth and adjust seasoning with more lime or fish sauce if needed, aiming for a bright, balanced flavor.
Garnish the soup with chopped cilantro and Thai basil, scattering herbs on top for vibrant color and fresh aroma.
Serve the soup hot in bowls, ensuring each has a good mix of chicken, herbs, and flavorful broth. Enjoy the bold, aromatic taste!