Place the turkey in a large glass or ceramic dish, breast side up.
Sprinkle the onion soup mix over the turkey, making sure the breast is well seasoned. Cover with plastic wrap and refrigerate for 2 to 3 days.
With the rack in the lower third position, preheat the oven to 350°F (180°C).
Place the turkey in a large roasting pan fitted with a roasting rack, breast side up. Tie the legs together with butcher’s twine. Pour the wine and 1 cup (250 ml) of the broth over the turkey, loosening the onion bits so they mix with the liquid in the bottom of the pan. Brush the turkey with oil.
Roast for 2 hours, gradually adding the remaining broth to the pan every 30 minutes, until a meat thermometer inserted into the thigh, without touching the bone, reads 180°F (82°C). Transfer the turkey to a large plate and keep warm.
Transfer the cooking juices to a pot and bring to a boil.
In a small bowl, whisk together the cornstarch and water.
Whisk the cornstarch mixture into the cooking juices and simmer for 3 to 5 minutes or until the gravy has thickened. Season with pepper. Serve with the turkey.