Preheat the oven to 350 degrees F (180 degrees C).
Melt butter over medium heat in a big pan, large soup pot, or stockpot. Combine onion, carrot, celery, and poultry spice in a large mixing bowl (and rosemary if using). Cook for 10-12 minutes over medium-low heat, until softening or tender (do not brown).
Combine chicken seasoning, rosemary, salt, and pepper in a mixing bowl. Cook for another 2 minutes. Turn off the heat.
Divide the work into two batches. Bread cubes should be placed in a big mixing dish or pot. Stir in the onion mixture, parsley, and fresh herbs until the seasonings and vegetables are uniformly distributed. Pour broth over the cubes until they are moist (but not mushy), then stir gently. It’s possible that you won’t need all of the broth. Toss in the egg until all of the ingredients are well coated. Season to taste with salt and pepper. Using a large baking dish, such as a lasagna pan, transfer the mixture to it. Wrap foil around the pan.
Transfer the mixture to a serving dish and top with more butter.
Preheat the oven to 350 degrees Fahrenheit and bake for 30 minutes. Bake for another 10 minutes with the foil removed. Serve hot right away, or cool, cover, and chill if you’re cooking it ahead of time. Before serving, warm the dish up again.