Peel the onion and cut into quarters. Peel the courgettes and potatoes and slice them into strips.
Place the potatoes and onions in a saucepan with 1 liter of water. Salt, pepper, cover and cook over medium heat for 10 minutes. Then add the courgettes and ½ liter of water, and simmer 10 more minutes.
Pour into the mixing bowl and mix to obtain a smooth soup. Add the olive oil and the rinsed basil leaves, then mix again so that all the ingredients mix.