Heat a pot over medium heat, add olive oil then add onions and saute until translucent, ~3 minutes.
Add broth and cauliflower and increase heat to medium-high. Bring to a boil, then lower heat, cover, and simmer until tender (~20 minutes).
Add the watercress and cook until wilted, ~1 minute.
Puree mixture (vitamix, immersion blender, or other blender) until smooth. Season with salt and pepper, add a squeeze of fresh lemon juice to brighten.