Generously salt fish and rinse under water to clean it. Set aside to dry.
Add oil to a pan and saute garlic over medium heat until lightly browned.
Layer onion on top, then the fish, evenly spaced.
Add the seasoning & cooking liquid (& optional chiles) and turn the heat to high until boiling. Taste the sauce and adjust the seasoning.
Bring it to a simmer for ~20 minutes. Adjust seasoning if needed.
Gently flip the filets of fish over and simmer for a final ~10 minutes with the lid partially covered.
Stand there to watch it the final ~10 minutes to prevent it from burning! During this time you can continually spoon the sauce over the fish.
Add green onion during last 2 minutes to wilt & top with sliced chiles, and some freshly ground black pepper.