Add the soy sauce to the skillet; when it sizzles, add the stock. Boil until reduced by half, about 6 minutes. Stir in 2 tablespoons of the mustard and reduce the heat to low. Return the pork medallions and any accumulated juices to the skillet and simmer until the pork is just pink in the center, about 1 minute. Stir in the parsley and turnips and cook until just heated through. Remove from the heat and stir in the remaining 1 teaspoon of mustard. Add the butter 1 tablespoon at a time and stir until the sauce is creamy. Season with salt and pepper. Transfer the medallions to plates, spoon the sauce, parsley and turnips on top and serve.