
In a wide pot over medium heat, heat oil. Add onions, garlic, and ginger, and cook until softened.
Add chicken and cook, stirring occasionally, until color changes and juices run clear.
Add fish sauce and continue to cook for about 1 to 2 minutes.
Add water and bring to a boil, skimming scum that floats on top.
Lower heat, cover, and cook for about 20 to 25 minutes or until chicken is cooked through.
In a bowl, mash hardboiled egg yolk with about 1/4 cup of the broth from the pot. Add to pot and stir to disperse.
Season with salt and pepper to taste. Continue to cook for another 1 minute.
Turn off heat and add chili leaves. Cover and allow residual heat to continue cooking the leaves until just wilted. Serve hot.