Preheat the oven to 375°F. Melt the butter in a medium skillet over medium heat. Add the onions and cook, stirring, for about 10 minutes; then add the apples and cook for 2 to 5 minutes or until soft and caramelized. Add the sage and then scrape the apple and onion mixture into a large mixing bowl. Add the cornbread pieces, season well with salt and pepper, and toss until well combined. In a separate bowl, whisk together the egg, heavy cream, and chicken stock and then pour 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.