Prep all of your ingredients.
In a pot on medium high, add the water, pepper, salt and onions. Allow it to come to the boil.
In a bowl, add the mince, pepper and a pinch of salt. With your hands, combine the ingredients together thoroughly. ( move the meat through your fingers!)
Break the mince into pieces and add it to the pot. Let the pot come back to the boil and skim off any scum with a spoon.
Add the chopped watercress and cook for roughly 2 - 3 minutes. And then turn off the heat.
Add 2 tbsp of fish sauce and adjust the soup according to your taste. If you want more seasoning + flavour, then add fish sauce otherwise add salt.
Serve by itself or with rice.