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.