Pistou is the Provencal counterpart to Italy’s pesto. It is made without nuts so it’s a bit lighter. One of the traditional uses for pistou is as a condiment for vegetable soups. You can add some sautéed broccoli raab to this soup, or half a cup of leftover beans. This soup is a great way to use leftover bits of vegetables.
Saute the onion in a tablespoon of olive oil until it has a little color, about five minutes. Heat the chicken broth in a small pot over medium heat with a cup of water. Season the broth with salt to taste. Cut the carrots and potatoes into ½” pieces. Add them to the pot along with the onions and turn the soup down to a simmer. While the vegetables cook make the pistou. Remove any large stems from the basil. Puree the basil, garlic and cheese in a food processor, adding enough olive oil so that it reaches a smooth consistency. Season with salt and pepper and set aside. When the vegetables are tender cut the spaghetti into 1” segments and add to the broth. Cook just until the spaghetti is al dente. Check the seasoning of the soup once more and divide into two bowls. Serve with a generous dollop of pistou.