Toast the bread slices in the oven or toaster, whatever you are comfortable with. While the bread is toasting, chop up the onion, parsley, celery, and apple and put everything in a large bowl. Add the seasoning, and add melted butter. Stir well.
When the bread is done, cut into ½ inch size-or-so cubes, add to the bowl and toss well to ensure the bread is evenly coated. Pour this into your slow cooker, pour in the chicken stock.
Cover and cook on High setting for 2 hours.
When finished, the bread will have browned a bit on the top and around the edges, and it will be hot throughout. It can stay on warm for probably another 2 hours before serving, if needed. Stir. If you'd like it a bit more moist, you can stir in a ¼ cup more (or as much as you desire) of chicken stock.