Use a 6-quart or larger slow cooker. Toast the bread slices in the oven on large baking sheets at 300ºF for 20 to 30 minutes. While the bread is toasting, add the onion, celery, apple, and parsley to the slow cooker. Add the melted butter and seasoning. Stir well. When the bread is done, cut it into ½-inch cubes. Add the bread to the slow cooker. Toss very well with the butter, chopped vegetables, and spices. When the bread is fully coated, pour in 1 ½ cups of broth. Cover and cook on high for 2 hours, stirring every 45 minutes.
When finished, the bread will have browned a bit on the top and around the edges. It will be hot throughout. It can stay on warm for 2 hours before serving, if needed, and can remain on the warm setting throughout the duration of your party.
Stir again before serving. If you’d like a bit more moisture, you can add ¼ cup more broth (or as much as you desire).