Heat the oil and butter over a medium heat then add the onion, celery and garlic.
Sweat under a lid until tender (stirring occasionally).
Add the carrots, stock and seasoning.
Bring to the boil, replace lid and then reduce to a simmer.
Cook for 10 minutes or until carrots are tender.
Stir in the coriander.
Blend the soup (I use a stick blender), adjust the seasoning if necessary and re-heat.
Serve garnished with some chopped coriander and some crusty bread.