Peel the onion and finely chop it. Scrub the carrot and finely dice it. Trim the celery stick and thinly slice it. Peel the garlic cloves and crush or finely chop them. Trim the tops off the courgettes and discard. Chop the courgettes into small pieces.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onion, carrot, celery, garlic and courgettes. Sprinkle in a little salt and pepper. Cook for 10 mins, stirring occasionally, till softened.
Crumble in the stock cube and pour in 750ml boiling water. Bring to the boil, then reduce the temperature to a simmer and cook gently for 5 mins.
Meanwhile, finely shred the kale leaves, slicing away and discarding the woody cores. Stir the kale into the soup and cook for a further 5 mins, till wilted.
Use a stick blender to whizz the soup till smooth. No stick blender? Transfer the soup to a blender (in batches if necessary), and blitz.
Have a taste of the soup and stir in a little more salt and pepper if needed. Ladle into a couple of warm bowls and serve.