Wash and clean the chicken and cut all the vegetables. (Note 1)
Optional Step: Lightly char the onion and ginger on your gas stove or put them in the oven at 200 C / 392 F on grill setting until it forms a slightly burnt layer. Remove any overly burnt bits before adding to the soup. Skipping this step WILL NOT ruin your soup.
In a large pot on medium high, bring 2.5 L of water to the boil and then add the whole chicken breast side down.
In a small fry pan on medium heat setting, toast the coriander seeds, star anise and pepper corns until fragrant. Keep the aromatics moving in the pan so that they do not burn.
Make a spice bag by cutting a square piece of cheesecloth / muslin / Chux wipes and place the toasted aromatics in the center. Bring the corners of the cloth towards the middle and tie it off with a string so that the aromatics cant escape. (Note 2)
Add the salt, sugar, spice bag and chopped vegetables to the pot. Once the pot is back to a boil, turn down the heat to medium low and allow to simmer for 50 minutes, Every now and then, skim the scum that accumulates on the surface.
After 50 minutes turn off the heat and allow the pot to sit for 10 minutes. Then remove the chicken from the pot and let it rest on a plate. Check the chicken to see if it is cooked. If not, pop it back into the pot and cook until done. (Note 3)
Strain the soup and press as much liquid from the vegetables as you can.
Add the fish sauce and check the soup for seasoning. Use more fish sauce to add more flavour or add more boiling water to dilute. (Note 4)