Sweat the chopped leek with a dash of extra-virgin olive oil in a pan. Once it has turned golden brown, add the courgettes and spinach and cook for a few more minutes. Add the potato and continue cooking. Add salt and pepper.
Add enough water the cover the vegetables and boil for about 25 minutes over a medium-high heat.
Check for salt. Pour off some of the liquid and put by in case you want to thin down the soup later. Blend the remaining the ingredients together and pour back some of the liquid if necessary.
Mix the extra-virgin olive oil with the balsamic vinegar and ground nuts in a bowl. Serve the soup in individual bowls and garnish with a spoonful of the vinaigrette.