Prepare the cornbread. Bake your favorite unsweetened cornbread, preferably 1–2 days ahead. If using White Lily Buttermilk Cornbread Mix, omit the sugar. Crumble the cooled cornbread into a large mixing bowl.
Preheat the oven. Preheat to 350°F (175°C). Grease a 9 × 13-inch baking dish.
Brown the sausage. Cook the sausage in a large skillet over medium heat, breaking it into crumbles, until browned and cooked through, about 5–8 minutes. Transfer to the bowl with the cornbread.
Sauté the vegetables. Melt the butter in the skillet over medium heat. Add the fresh sage leaves, if using, along with the diced onion and celery. Cook for about 5 minutes, or until the vegetables soften. Remove and discard the sage leaves, then transfer the vegetables and melted butter to the mixing bowl.
Mix the dressing. Add the diced apple, undiluted cream of celery soup, beaten eggs, stock, salt, pepper, ground sage, and poultry seasoning. Gently stir until evenly combined. The mixture should be very moist without liquid pooling in the bowl. Add a little more stock if needed.
Bake. Transfer the dressing mixture to the prepared baking dish and spread evenly. Bake for approximately 45 minutes, or until golden on top, set in the center, and the internal temperature reaches 165°F (74°C). Loosely cover with foil if the top browns before the center finishes cooking. Optional: I like to save back a few chopped pieces of the cornbread and some of the sausage crumbles to sprinkle on top before baking.
Serve. Let the dressing rest for 5–10 minutes before serving.