Trim the fennel bulb and slice thinly, reserving the fronds for later.
Thinly slice the onion.
Sweat the fennel and onion in olive oil with a pinch of salt for 10 minutes, then add the butter beans and the liquor from the tin and the vegetable stock. Bring to a simmer and cook gently for 20 minutes.
Blitz with a stick blender until smooth. Taste and season with salt and pepper.
