Preheat the oven to 375 degrees F and remove the top rack of the oven.
Rinse the bird thoroughly inside and out with cold water, and pat dry with paper towels. Sprinkle the cavity and skin liberally with salt and pepper. Stuff the cavity with stuffing and, if required, truss the legs. Cover the turkey with olive oil and season well with salt and pepper. Push the sage butter under the skin of the turkey, being careful not to puncture the skin.
Put the turkey on a rack in a large roasting pan, and into the oven. Continue cooking until an instant-read thermometer inserted into the meaty part of the thigh registers 170 degrees F. The thigh juices will run clear when pricked with a knife, about 3 hours total (15 minutes per pound). If the legs or breast brown too quickly during roasting, cover them with foil.
Combine all ingredients.
Melt the butter in a medium skillet over medium heat. Add the onions and cook, stirring, for about 10 minutes, or until soft and caramelized. Add sage and scrape into a large mixing bowl. Add the cornbread pieces, season well with salt and pepper, and give it a good toss until it's well combined. In a separate bowl, whisk together the egg, cream, and stock, and pour that over the cornbread. Stir the stuffing together and stuff the cavity of the turkey. You could also spoon it into a buttered baking dish and put it in the oven along with the turkey. Bake until hot and crusty on top, about 30 minutes.