19 kg/20 lbfresh whole turkey (thawed, if using frozen)
Freshly-ground black pepper
1/3cupall-purpose flouror more, as needed
1litre unsalted chicken brothcold or room temperature
Combine the first 12 ingredients in a food processor or blender and process until smooth.
Remove fresh turkey from bag. Remove neck from body cavity and giblets from neck cavity. Place turkey, breast side up, on a rack in a shallow (3" deep) roasting pan. Slather the yogurt-spice rub all over the turkey, sprinkle with black pepper, and let stand at room temperature for 1 hour. Chop the onion in half and place it in the cavity.
Preheat oven to 325ºF. Position turkey on centre oven rack and roast until meat thermometer reads 180°F (82°C) in the thigh, about 4 hours. About half-way through cooking time, tent the turkey with foil if the skin is getting dark. Transfer the turkey to a cutting board and allow it to rest for 20 minutes before carving.
Meanwhile, make the gravy. Place the roasting pan with all of the turkey bits and juices on the stove over medium heat. Place the flour in a bowl. Slowly drizzle one cup of the cold chicken broth into the flour, whisking constantly to make a smooth mixture without any lumps. Gradually whisk this flour mixture into the stove-top simmering juices, followed by the remaining broth and the butter. Allow the gravy to come to a boil then reduce the heat and simmer until thickened. Taste and add salt as needed. Strain into a serving bowl.