Heat in a large soup pot over medium heat:
Add and cook, stirring, until tender but not browned, about 8 minutes:
- 1 onion, chopped
- 1 small leek, halved lengthwise, well rinsed, and chopped
- 1 carrot, chopped
- 1 large celery rib, chopped
Stir in:
- 2 medium tomatoes, peeled, seeded, and chopped
- 1 small red or gold potato, chopped
- 8 cups water
- 1 teaspoon salt
- (Pinch of saffron threads)
Bring to a boil, reduce the heat, and simmer until the potatoes are tender, about 30 minutes. Stir in:
- One 15-ounce can cannellini or other white beans, drained and rinsed, or 1 ½ to 2 cups cooked beans
- 1 cup broken thin spaghetti or elbow macaroni (4 ounces)
- 1 small zucchini, halved lengthwise and sliced
- 1 cup 1-inch pieces green beans
Simmer just until the pasta is tender. Meanwhile, prepare without adding salt or pepper:
Stir the pistou into the soup along with:
- 1 teaspoon black pepper
- Salt to taste
Serve hot, at room temperature, or cold.