Heat the olive oil in a large, heavy soup pot over medium heat and add the onion, leeks and a pinch of salt. Cook, stirring, until tender, about 5 minutes. Add the garlic and cook, stirring, until fragrant, 30 seconds to a minute. Add the turnips, potatoes, water or stock, salt to taste, and the bouquet garni. Bring to a boil, reduce the heat, cover and simmer 45 minutes, or until the vegetables are very tender and the soup is fragrant. Remove and discard the bouquet garni.
