Heat the olive oil in a large pot and add the onion, leek and a pinch of salt. Cook, stirring, until tender, about 5 min. Add the carrots, parsnips and a generous pinch of salt. Cover partially and cook for another 5 - 10 min., stirring often until the vegetables are tender and fragrant. Add the potatoes, liquid, salt to taste and the bouquet garni. Bring to a boil, reduce the heat, cover and simmer 45 min., or until the vegetables are very tender and the soup is fragrant. Remove and discard the bouquet garni.