For the turkey: Preheat the oven to 425 degrees F. Remove the turkey from the refrigerator 30 minutes before roasting.
Rub the cut side of 2 halves of garlic over the entire turkey, and then put in the cavity. Brush the entire turkey with 4 tablespoons of the butter and sprinkle with the rosemary, salt and pepper (including in the cavity). Put 2 lemon halves into the cavity.
Put the shallots, carrots, celery and remaining garlic and lemon halves in the bottom of a roasting pan. Place a roasting rack in the pan and place the turkey on the rack. Add 2 cups of the stock to the bottom of the pan and place in the oven. Put the remaining 4 tablespoons butter and 4 cups stock in a medium saucepan and bring to a simmer over low heat. Keep covered and warm.
Roast the turkey for 30 minutes. Reduce the heat to 375 degrees F and continue roasting, basting with the warm stock every 20 minutes, until an instant-read thermometer inserted into the breast registers 155 degrees F and into the thigh registers 160 degrees F, about 2 1/2 hours. Remove the turkey to a cutting board, loosely tent with foil and let rest at least 20 minutes before carving.
Strain the juices into a bowl through a strainer. Let the fat rise to the top of the stock and remove 1/4 cup of the fat. Reserve the fat to make the gravy. Reserve the strained juices for the gravy.
For the gravy: Heat the reserved fat in the roasting pan (used to roast the turkey) over high heat over 2 burners. Add the mushrooms and cook until golden brown and the liquid has evaporated, about 8 minutes. Stir in the flour and cook until pale golden brown. Whisk in the red and white wine and cook until evaporated. Add the reserved roasting liquid and chicken stock and bring to a boil. Add the rosemary sprigs and cook until the gravy thickens, stirring constantly, about 5 minutes. Stir in the parsley and season with salt and pepper.
Carve the turkey and serve with the cremini mushroom gravy.