Wash the courgettes, trim the ends and cut into medium chunks, leaving the skins on if they’re tender.
Peel the potato and cut into chunks of a similar size to the courgettes.
Peel and finely chop the onion.
Heat the olive oil in a medium saucepan over medium heat.
Add the onion and cook for 5–7 minutes until softened and translucent.
Add the courgettes and potato, stir through and cook for a further 5 minutes.
Pour in the water or vegetable stock, season with salt and freshly ground black pepper.
Bring to the boil, then reduce the heat, cover and simmer for 15–20 minutes, or until the vegetables are tender.
Remove from the heat and blend with a hand-held blender until smooth.
If the soup is too thick, stir in a little more water or stock until it reaches your preferred consistency.
If you like, stir in the milk and blend again to combine.
Serve immediately, drizzled with a little extra virgin olive oil and topped with crispy croutons and fresh parsley.