How to Make It
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.
Add the chicken breasts to the simmering stock and poach at a bare simmer until just cooked through, about 12 minutes. Transfer the chicken to a plate and let rest for 5 minutes. Return the vegetables to the stock to reheat them; season with salt and pepper.
Remove the skin from the chicken and slice each breast diagonally 1/3 inch thick. Divide the vegetables among 8 shallow bowls. Arrange the chicken over the vegetables and add the hot broth.