Preheat the oven to 350°F and remove the top rack of the oven.
Put the butter and sage in a mixing bowl and mash with a fork or spoon until the sage is well incorporated. Season with salt and pepper. Put about half of that in a sauté pan and melt over medium heat. (Reserve the rest for the turkey.) Add the onions and cook for 10 minutes or until soft and golden. Scrape that all 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 1/2 cup stock, and pour that over the cornbread. Stir the stuffing together and set it aside while you work on the turkey.
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. Using your fingers, gently lift the skin from the turkey breast and slip the remaining seasoned butter under, massaging the breast meat as you go. Stuff the bird with the cornbread stuffing and truss it (or just cross the legs over one another and tie with a piece of kitchen twine). Put the turkey on a rack in a large roasting pan, and into the oven.
In a small bowl, stir the maple syrup with 2 tablespoons of hot water to thin. Roast the turkey for 2 hours, basting with the maple glaze every 30 minutes. Then pull the oven rack out a bit so you can get at the turkey and shingle the bacon strips over the breast so it’s totally covered. Continue cooking until an instant-read thermometer inserted into the meaty part of the thigh registers 170°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. Take the turkey out of the oven and put the roasting pan on the stovetop. Transfer the turkey to a serving tray to rest while you make the gravy.
Skim off most of the fat from the pan drippings with a spoon and put the roasting pan across two burners, over medium-high heat. Using a wooden spoon, scrape up the browned bits stuck to the bottom of the pan. Whisk in the flour, stirring as it thickens to prevent lumps. Pour in the remaining 2-1/2 cups stock, 1/2 cup at a time, stirring constantly to blend and prevent lumps. Bring to a simmer, season with salt and pepper and a squeeze of lemon juice to brighten the flavors. Simmer for 5 minutes and then strain to remove the bits. Carve the turkey and serve with the gravy and stuffing.