Preheat the oven to 350 degrees F. Coat the bottom and sides of a 9-by-5-inch loaf pan with cooking spray and set aside.
In a large skillet, add the butter and melt over medium heat. Add the celery, onions, thyme, sage, scallions, a pinch of salt and a few grinds of pepper. Cook until the onions and celery are tender, 8 to 10 minutes. Add the garlic and cook for another 30 seconds to a minute. Transfer the vegetables to a large mixing bowl.
To the bowl of veggies, add the stuffing mix, walnuts, parsley, cranberries and apples and mix well. Add 2 1/2 cups of the stock, tossing well to coat and soak all of the cubes. If more stock is needed, add it splash by splash. There should be no liquid pooling in the bottom of your bowl.
Scoop the stuffing mixture into the prepared loaf pan and press down firmly to pack it. Cover the pan with foil, place on a sheet pan and bake for 1 hour. Increase the oven temperature to 400 degrees F. Remove the foil and continue to bake until golden brown on top, 25 to 30 more minutes.
Dig in warm or allow to cool slightly and turn out onto a plate to serve. Or allow to cool completely, then cut into 1-inch-thick slices with a serrated knife and toast in a pan with butter.