Melt the butter in a large saucepan over a medium heat, add the onion and a large pinch of salt (the salt stops the onions from browning) and saute gently for 5 minutes. Add the garlic, courgette and peppers and continute to saute for a further minute.
Add vegetable stock to the pan so that it comes to a level approximately 1cm below the level of the vegetables. [Courgettes and peppers are full of water and vary greatly in size, by doing it this way rather than stipulating a quantity of stock, you will avoid having a watery soup. You can always add more stock later if you need it, but you can't take it away!]
Simmer the soup for 10 minutes, stir through the cream and season to taste with salt and black pepper. Blitz the soup using a stick blender and serve.
If following a Vegan or dairy free diet, simply substitute the butter with olive oil and remove the cream entirely as the soup is lovely without it.