Preheat the oven to 180° C (360° F).
Pat dry the turkey leg with a kitchen towel, rub it with half of the salt and pepper, and set it aside.
Prepare the vegetables. Peel the onion and cut it into 5-6 wedges. Peel and cut the carrot into chunks. Wash the celery stick and cut it into 4-5 pieces. Cut the garlic head in half.
Place the vegetables along with the bay leaves and thyme in your baking dish. Sprinkle on the rest of the salt and pepper, and give them a good stir.
Pour 250 ml chicken or turkey stock on the vegetables and place the turkey leg(s) on top of the vegetables.
Place the baking dish into the preheated oven and roast the turkey for 40 minutes.
Reduce the heat to 150° C (300° F) and keep roasting the turkey for another 2 hours, or until the meat is fork tender. Flip it halfway through the cooking.
When the turkey meat is cooked, remove it from the baking dish, wrap in tin foil, and leave it to rest for 15 to 20 minutes while preparing the gravy.
To make the gravy, skim most of the fat from the juice of the roasted turkey and place the baking dish on medium-low heat. Add 250 ml of chicken or turkey stock and scrape the bits off the bottom of the pan.
Let the gravy bubble for 15 minutes, pass it through a fine sieve, and transfer it into a small saucepan.
Place the saucepan on medium heat, and bring it back to a boil.
Make a slurry with 1 tablespoon of cornflour and 2 tablespoons of cold water, and add it to the gravy, whisking continuously to avoid the gravy getting lumpy.
Add any juices from the resting turkey and then cook for about 5 more minutes.