Heat the olive oil in a large pot over medium heat. Add the onion and cook until softened, about 5 minutes.
Add the garlic and cook for another minute until fragrant.
Pour in the vegetable broth and bring to a simmer.
Add the watercress and cook until wilted, about 3 minutes.
Carefully transfer the soup to a blender and blend until smooth. (Alternatively, use an immersion blender).
Return the soup to the pot and season with salt and pepper to taste. Heat through gently.
Serve hot.
Notes
Garnish with a swirl of vegan cream or a sprinkle of fresh herbs if desired. For a creamier soup, add a can of drained and rinsed white beans before blending.