In a large bowl, mix the stuffing mix with the parsley and set aside.
In a large pan on medium heat, add the butter, celery, onions, thyme, sage and garlic. Season with a pinch of salt and a nice grind of pepper. Cook until the onions and celery are tender but the garlic is not browned. Remove to the bowl of bread cubes. Pour the stock over the bread and veggie mixture and stir to combine (this is a wet dressing, so it should be very moist). If using one of the add-in combinations, mix in the ingredients carefully.
Pour the mixture into a 13-by-9-inch baking dish. Press down to make it compact, and then cover tightly with aluminum foil. Bake for 45 minutes, then raise the oven temperature to 400 degrees F and bake, uncovered, until golden brown on top, about 15 minutes more.