1. Place oil in a large deep skillet that can later be covered and turn heat to medium. A minute later, add the turnips, salt, and pepper and cook, stirring occasionally, until turnips begin to brown, about 10 minutes. 2. Add the stock, cover and simmer until the turnips are tender, 10-15 minutes. 3. Remove the turnips to a serving bowl with a slotted spoon; keep warm. Mix the cornstarch into the mustard and stir the mixture into the pan juices. Cook over low heat until lightly thickened, a minute or two longer. Pour the sauce over the turnips, garnish and serve.