Chop the fennel and onion. Peel and chop the potatoes into small chunks.
Heat the olive oil in a heavy-based saucepan over a medium heat. Add the fennel and onion and cook for about 8 minutes, until softened but not browned.
Add the potatoes, stock, bay leaf and thyme. Bring to the boil, then reduce the heat and simmer for about 20 minutes, or until the potatoes are tender.
Remove the bay leaf and thyme. Season with salt, if needed, and freshly ground black pepper.
Using a hand blender, purée the soup until smooth.
Add the watercress and blend again for just a few seconds, until incorporated but still fresh and vibrant.
Serve warm, garnished with a few fresh watercress leaves and a dollop of Greek yoghurt or plant-based cream.