Cut the courgettes into slices about 1 cm thick and then quarter them.
Heat the oil and butter in a large saucepan.
Add the onion, garlic and courgettes, stir well and cook gently for about 10 minutes with the lid on, stirring occasionally.
Add the vegetable stock and simmer for a further 10 minutes.
Allow to cool a bit and then blend until smooth.
If you want it thinner, you can add some extra water at this point.
Add the sweetcorn and reheat gently making sure that the sweetcorn and the soup are both hot.
I like this soup just as it is but you might want to season it with salt and black pepper.