Wash the watercress and rinse several times. Remove as much of the stems as you can. It's okay to eat the stems but stems make the soup more bitter.
In a large pot, saute onions and garlic for a few minutes. Add broth, potatoes, curry powder, ginger, salt and pepper. Bring to a boil, then reduce heat and simmer for about 15 minutes until the potatoes are soft.
Add watercress and simmer another 5 minutes. Turn off the stove. After a few minutes, you can mix in the coconut milk.
Using an immersion blender, blend the soup until smooth.
Optional: Garnish with croutons or crumbled baked pita chips
