I love soups and used to cook this quite often in UK.
The watercress sold there were washed fine leaves ready to eat as Salad. Sometimes I felt that there were not enough roughage e.g. stems in it for cooking for long periods.

I’m not sure why I waited so long to buy this but it is my first time buying it here in Malaysia.
Watercress Soup Recipe
Ingredients
Watercress, Red dates, Water, Pork bones
Method
Boil the pork bones for a few minutes
1. Wash it
2. Add pork bones and red dates into a slow cooker/ pot

3. Set it to simmer in pot/ high in slow cooker
4. After half an hour, add in watercress

5. Simmer/ cook in slow cooker for a couple of hours.