Wash the courgettes and slice them with peel on into 2cm thick rounds.
Place the vegetable oil and the butter in a pan over low heat and when the butter is melted add the courgette rounds. Let them cook in the butter and oil for about 10 minutes, stirring occasionally until the courgette looks soft in the centre.
Dissolve a chicken stock cube (or vegetable stock cube if you are vegetarian) in 500ml of boiling water and add to the pan (my favourite is Bouillon).
Mix well and add another 500ml of boiling water. Let it simmer in low heat for 30 minutes covering the pan loosely with the lid.
After the 30 minutes, remove the lid and let it simmer another 10 minutes.
Remove the pan from heat and add the cheese.
Blitz everything with a handheld blender and add salt and freshly ground black pepper to taste.
For the croutons, cut the sourdough into small cubes and fry on a frying pan with a little olive oil and butter, until they are golden brown. Add a few to each soup bowl just before serving. Delicious!