In a medium pot melt the butter. Add roughly chopped onion and sauté for 3 – 4 minutes.
Meanwhile wash and roughly cut broccoli and celery. Use all parts of broccoli including the stems. Throw the broccoli stems and celery in the pot. Pour in 1 liter of vegetable stock / water. Season with salt.
Bring to the boil, reduce the heat to low-medium, cover with lid and simmer for 10 minutes.
Now add the green parts – florets to the pot and simmer for a further 10 minutes or until the broccoli has soften.
Turn off the heat and puree it until smooth using a hand blender.
Taste and season with black pepper and some more salt if required.
Serve in a bowl garnished with a handful of croutons (optional).
TIP: If using water, add vegetable stock cube for extra flavor.