Preheat the oven to 270°C (250°C fan).
Line an oven proof dish with roasting foil.
Then, place a large piece of roasting foil in the dish.
Put the turkey leg on top of this roasting foil.
Evenly spread the rosemary, olive oil and salt (if you choose to add this) over the turkey leg.
Bring up the sides of the roasting foil, so they completely cover the turkey leg. Scrunch the foil at the top to seal the turkey leg in a pouch.
Cook the turkey leg in the oven for 1 hour.
Take the turkey leg out of the oven and partially open the top of the roasting foil, so that most of turkey leg is exposed.
Spoon some of the excess oil that the turkey sits in over the top of the leg.
Place the turkey back in the oven and cook for another 1 hour, this time at 220°C (200°C fan).