In a large saucepan heat the olive oil. When hot, add the onions, leeks, carrots and celery. Cover and simmer for 5 minutes or until tender; if at any time the vegetables begin to stick to the bottom of the pan before they become tender, add some water.
Add the tomatoes, saffron and water; bring to a boil and simmer uncovered for 15 minutes or until the vegetables are tender. Season to taste with salt and pepper. Divide the soup base into 2 containers and freeze until you are ready to use.
Bring soup base to a simmer (if there does not appear to be enough liquid in which to simmer all of the additional vegetables, then add some water). Add potatoes or white beans, zucchini and pasta, cover and simmer for 10 to 15 minutes or until all the ingredients are tender. Serve as is or with pistou or rouille (see below).