Up to a day ahead, make the stuffing. About 2 hours before serving, blanch the cabbage leaves for 5 minutes in boiling salted water. Rinse under cold running water and drain. If the ribs are very thick, shave them off with a knife. Enclose each chicken breast in ⅓ cup of the stuffing. Wrap each breast in 2 cabbage leaves, rib sides out, and tie securely with string. (If desired, wrap each cabbage roll in a 12-inch square of cheesecloth and tie with string.) About 1 hour before serving, bring the stock to a boil in a 5-quart casserole. Add the leeks, carrots, and turnips and simmer until tender, about 20 minutes. Remove the vegetables and set aside. Add the cabbage rolls to the simmering broth and poach them over low heat for 25 minutes. Remove the cabbage rolls to a work surface and let them rest for 5 minutes. Remove the strings (and cheesecloth, if used) from the cabbage rolls. Meanwhile, reheat the vegetables in the stock; drain. Cut each cabbage roll into 4 slices slightly on the diagonal. Arrange the slices cut sides up, on warmed plates. Sprinkle the chicken with fleur de sel. Surround each serving with assorted vegetables; moisten with a few tablespoons of the cooking liquid. Drizzle a little walnut oil over the chicken and vegetables and serve at once. Trim any yellow fat or green spots from the chicken livers. Soak them in ⅓ cup of the milk with the salt in the refrigerator for at least 3 hours. Drain, rinse, and pat dry. In large saucepan of boiling salted water, blanch the Swiss chard leaves (save the stalks for another use) for 1 minute. Drain, rinse under cold running water until the water runs clear, and squeeze dry. Finely chop the Swiss chard. In a food processor, combine the chicken livers, hearts, gizzards, ham, and ventrèche. Pulse until coarsely chopped. Add the onion, shallot, and flour and process until well blended. In a mixing bowl, soak the bread in the remaining ⅓ cup milk for a few minutes to soften. Press out and discard the excess moisture; leave the bread in the bowl. Add the egg and mix with a fork until they are light and well combined. Mash in the chicken liver mixture, then add the Swiss chard, chives, and parsley. Season with the salt, pepper, nutmeg, and sugar. Cover and refrigerate the stuffing for at least 2 hours or overnight to mellow and firm up.