Pre-heat oven to 350 degrees Fahrenheit and grease a 12" round cast iron pan with butter. (You can also use an alternate pan such as a rectangular 9x13 pyrex baking dish.)
Sauté the celery and onions in the butter with the poultry seasoning and salt until the celery is soft and the onions are translucent, but not browned. Approximately 10 minutes.
While the celery and onion cook, add both types of bread cubes to a large bowl.
Once the celery and onion are cooked, pour the celery, onion, and butter mixture over the bread cubes and mix well.
Add the eggs to the milk and mix well, then pour over the bread cube mixture. Mix well until thoroughly combined.
Pour the stock or bone broth over the bread cube mixture and mix well.
Transfer the entire mixture to the greased pan and place on the middle rack of the pre-heated oven. Allow to bake for 30-40 minutes until the top is a crispy golden brown.