Warm 1 tablespoon of the oil in a large pot. Sweat the onion, add the leeks, and season with salt and pepper to taste. Add the carrots, potatoes, zucchini, chicken stock, and spinach. Bring to a boil, then cover and simmer for 20 minutes, until all the vegetables are tender.

Purée in a food mill and serve with a drizzle of remaining oil and the basil, if using.

Note: In the summer, this soup can be served cold. Remove it from the refrigerator 20 minutes before serving. Add a drizzle of lemon juice and replace the basil with fresh mint or cilantro.