In a large saucepan, bring the chicken stock to a simmer. Add the carrots and cook over moderate heat until tender, about 10 minutes. Using a slotted spoon, transfer the carrots to a plate. Add the turnips to the stock and cook until tender, about 6 minutes. Add to the plate of carrots. Simmer the scallions for 5 minutes and add them to the plate. Simmer the mushrooms just until tender, about 4 minutes; add to the plate.