In a Dutch oven or soup pot, melt the butter.
Add onion, season with salt and pepper, and cook over medium heat stirring frequently until the onion is translucent, about 5 to 6 minutes.
Add the water and diced potato and cook, partially covered, until the potato cubes are tender, about 10 minutes.
Turn off heat and add the watercress and peas, stirring a few times to wilt the cress.
Puree the soup using an immersion blender, or wait until the soup is lukewarm and puree until smooth in a blender.
Once the soup is pureed, rewarm the soup in the pot, season with salt and pepper and serve.
3 tablespoons butter (replace with vegan butter to make a vegan dish)
1 medium onion, peeled and diced
Salt and freshly ground black pepper to taste
6 cups water
1 pound potatoes peeled and diced
8 ounces watercress, leaves and tender stems
1 cup fresh or frozen peas