heat the olive oil in a large stockpot, add the onions, and sauté over low heat for 10 min or till the onions are translucent.
Add the leeks, potatoes, carrots, salt and pepper and sauté over med heat for another 5 minutes add the chicken stock and saffron, bring to a boil then simmer uncovered for 30 min or till all the vegetables are tender. Add the haricots verts and spaghetti, bring to a simmer and cook for 15 more minutes.
To serve whisk ¼ cup of the pistou (see note above) into the hot soup, then season to taste.
Depending on the saltiness of your chicken stock, you may need to add up to another T of salt.