In a pot, heat 2 Tbsp of olive oil over medium heat. Add onions & cook until softened, 5 minutes, stirring occasionally. Add minced ginger, cook for 30 seconds, stirring frequently.
Add potatoes, broth, water ,a nd a pinch of salt. Cook covered 10 minutes until potatoes are soft.
Turn off heat and add watercress, peas, scallions, and basil leaves. Keep stirring until the watercress has wilted. Let everything cool in the pot for 10 minutes before blending with an immersion blender or mixing in a blender.
Pour pureed sop back into the pot to reheat. Season with more salt and pepper, and serve with a splash of coconut milk, if desired.