Season: Pat dry the turkey leg. Cut it around the joint, if necessary. If your Dutch oven (or similar pot) is large enough, you don't have to do that. Rub the leg with the mustard and sprinkle with salt and pepper. Place the turkey in the Dutch oven or cast iron oven dish.
Assemble the dish: Quarter the onions and chop all the vegetables into large pieces. Peel but leave the garlic cloves whole. Place them and the bay leaves in the pot around the turkey leg. Pour in the chicken stock, taking care not to wash away the mustard. Just pour around the leg.
Roast: Place the pot in the cold oven, turn the temperature to 150 degrees Celsius/ 300 degrees Fahrenheit. Cook for 3 ½ to 4 hours until the skin is crisp and the meat very tender.
Check from time to time to ensure that the skin doesn't get too dark. If that starts to happen, place a piece of aluminum foil very loosely on top of the pot, just place it on top without pressing it or trying to close the pot.
Rest: Remove the turkey leg from the pot and place it on a warmed plate, cover it loosely with aluminum foil and let rest while you make the sauce.
Gravy: Puree the vegetables in the pot, adjust the taste with salt and pepper.
Serve immediately with potatoes and vegetables.