2) Add the leeks, potatoes, carrots, salt and pepper and saute over medium heat for another 5 minutes. Add the chicken stock and saffron, bring to a boil and simmer uncovered for 30 minutes, or until all the vegetables are tender.
3) Add the green beans and spaghetti, bring to a simmer, and cook for 15 minutes more.
For the pistou:
1) Place the garlic, tomato puree, basil and parmesan in the bowl of a food processor and puree.
2) With the motor running, slowly pour the olive oil down the feed tube to make a paste. To store, pack into a container, pour a film of olive oil on top, and close the lid.
To serve, whisk 60ml of the pistou into the hot soup, then season to taste. Depending on the saltiness of your chicken stock, you may need to add up to another tablespoon of salt. Serve with grated parmesan cheese and the remaining pistou.