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. Scrape into a large mixing bowl, add the cornbread pieces and the sage, 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, spoon it into a buttered 9 by 13-inch baking dish and put it in the oven along with the turkey. Bake until hot and crusty on top, about 30 minutes.