Soak the kiam chye in a basin of warm water for about 20 minutes (water should cover 2 inches above the kiam chye). Taste the kiam chye to see if it is too salty. If so, continue to soak for another 10 minutes.
De-skin the duck and remove all the fats. Removethe innards and wash thoroughly, then chop the duck into chunks.
Blanche the duck in boiling water for 3 mins, then rinse with running water to remove any remnant blood clots.
Separate out the kiam chye leaves. Next, cut off the fibrous stem and discard, then cut the leaves into smaller pieces.
Wash and cut the tomatoes into wedges.
Next, wash the tamarind skin and the sour plums.
Lastly, peel the ginger and lightly bruise it with a knife.
Boil 3 litres of water in a large pot. Once boiling, add in the duck, tamarind skin and ginger and continue to boil over medium heat for half an hour.
Turn down the heat to medium-low, then add in the kiam chye, tomatoes and sour plum. Boil for another 30 mins or until the duck is soft but not disintegrated. Best served piping hot!
Avoid buying pre-packed kiam chye if possible. You can get fresher kiam chye from the wet market or those from Sheng Siong, where the kiam chye is soaked in a big tub of salty brine.
Don't cut the duck or kiam chye too small or it will disintegrate very quickly during cooking.
Don't change the water when soaking the kian chye, or the kiam chye will become tasteless, as the saltiness of the kiam chye will keep being absorbed into the fresh water.
Ensure all the fats of the duck are removed, otherwise the soup will be very oily.
3 red chilli padi (sliced lengthwise and seeds removed) and 1 TBsp of brandy can be added if that is your preference.
I do not recommend this soup for young children because of the high sodium content from the kiam chye.
HOW TO REMOVE THE OIL FROM THE SOUP?
If the soup is still oily, there are a few methods to remove the oil.
You can put it in the fridge and just scoop off the fat layer that coagulates on top. I find it troublesome because then you have to cook the soup the day before for the method to work, and it takes up space in the fridge.
I use my oil separator / fat separator, which is one of my best kitchen devices that I have used. You just have to pour the soup into the container, and pour the soup out, leaving the oil behind. The oil, being lighter, will float on top of the soup. Its easy to use and easy to clean as well. You can pick up one from Amazon for less than US$15.