Place the oil in a large, deep skillet that can later be covered and turn the heat to medium. A minute later, add the turnips, salt, and pepper and cook, stirring occasionally, until the turnips begin to brown, about 10 minutes.  Add the stock, cover, and simmer until the turnips are tender, 10 to 15 minutes.  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.