Prepare your watercress by pulling off any thick stems saving the leaves and thin tender stems only.
Bring 4 cups of water to a boil in a teakettle.
Heat oil in medium sized pot and add in onion, garlic, and potatoes. Season with salt and pepper and cook for 8-10 minutes or until your potatoes are fork tender. If the pot gets too dry just add some water as needed.
Add the watercress to the pot and top with 4 cups of boiling water let this simmer for a couple of minutes the goal is for the watercress to be wilted but still vibrant.
Allow everything to cool down and then pour soup into a blender, no more than halfway, you may have to work in batches.
Blend on high until your soup is smooth and velvety. Season the soup with salt, pepper and sesame oil. Repeat with any remaining soup. Enjoy!
Makes 5 cups.