How to Make It
In a bowl, combine the mustard, olive oil, garlic, rosemary, salt and pepper and blend well. Rub the mixture all over the turkey. Arrange the breasts on a large rimmed baking sheet and the wings and legs/thighs on another baking sheet. Refrigerate uncovered overnight.
Preheat the oven to 450° with the racks set in the upper and lower thirds. Let the turkey stand at room temperature for 30 minutes.
Divide the onion and wine around the turkey on the baking sheets. Roast the white meat in the lower third of the oven and the dark meat in the upper third for 30 minutes. Reduce the oven temperature to 350° and roast the white meat for 20 minutes longer and the dark meat for 30 minutes longer, until an instant-read thermometer inserted in the thickest part of the meat registers 165°. Discard the onion. Let the turkey rest for 15 minutes before carving. Serve with the port gravy.
To make the Port and Black Pepper Gravy even more delicious, you can strain the turkey pan juices here and skim the fat, then whisk the juices into the gravy when you add the port and stock.