Heat the remaining 1 tablespoon of olive oil in the casserole. Add the turnips and carrots, season with salt and cook over moderate heat for 1 minute. Add 1 cup of the stock, cover and cook over low heat, stirring occasionally, for 15 minutes. Add another 1/2 cup of the stock, cover and cook until the turnips and carrots are tender, about 10 minutes longer. Add the zucchini and 1/2 cup of the stock and simmer until the zucchini is just tender, about 4 minutes. Add the remaining 1/4 cup of stock to the casserole along with the lemon zest, lemon juice and lettuce and cook, stirring, until the lettuce just wilts, about 20 seconds. Stir in the crème fraîche.