Prepare the cornbread: Spray or wipe a 8×8-inch baking dish with oil. Preheat oven to 400F. In a large bowl, mix the cornmeal, flour, baking powder, cornstarch, and salt well. In a smaller bowl, mix the milk, apple sauce, and 1/4 cup water. Add wet to the dry and stir gently until evenly distributed, but don’t over-mix. Pour into the prepared pan and bake for 20-30 minutes. When a knife blade comes out clean, it’s done. Set aside to cool. (You can do this a day or two ahead of time.)
Once the cornbread is cool, crumble it into a very large bowl. It does not have to be completely broken down into crumbs, but there should be few pieces larger than 1/2 inch. Tear the whole grain bread into small pieces (you can do this in two batches in the food processor, if you like) and add it to the cornbread.
Heat a large non-stick skillet. Add the onions and celery and a splash of water and cook until the onions begin to soften, adding water by the tablespoon if necessary to prevent sticking. Add the mushrooms and garlic, a tablespoon or two of water, and cover. Cook until the mushrooms soften and shrink, about 4 minutes.
Add the vegetables to the crumbled bread. Stir in the thyme, sage, rosemary, and black pepper, making sure the seasonings are evenly distributed.
Stir the soy sauce and nut butter, if desired, into the hot broth. Pour it over the bread mixture, stirring as you go. Bread should be thoroughly moistened but not swimming in liquid. Check the moistness and add additional broth as needed. (I used an additional 2/3 cup broth.) Check the seasonings and add salt and pepper to taste. Sprinkle with a little smoked salt, if desired.
Preheat oven to 350. Oil a large, 9×13-inch baking dish. Transfer the dressing to the baking dish and smooth the top without packing it into the dish. Cover with foil and bake for 30 minutes. Remove the foil and bake for 20-30 more minutes, until top is brown but dressing is not dried out. Serve hot with Mushroom Gravy.