Preheat a large skillet over medium-high heat. Melt 1 stick butter in the skillet. When the butter melts, add the celery, apples and onions. Season with salt, pepper and poultry seasoning. Cook until the vegetables and apples begin to soften, 5 to 6 minutes.
Add the stuffing cubes to the pan and combine. Moisten the stuffing with the chicken stock until all of the bread is soft, but not wet. Let the stuffing cool either in the pan or in a bowl.
For a stuffin' casserole: Brush a casserole dish with 1/4 stick butter. Add the stuffing mixture to the dish and dot the top with the remaining 1/4 stick butter. Bake until set and crisp on top, 15 to 20 minutes. Serve hot or at room temperature.
For individual stuffin' muffins: Brush a 12-muffin tin with the remaining 1/2 stick of butter. Add the beaten egg to the cooled stuffing mixture. Using an ice cream scoop, fill and mound up the stuffing in the muffin tin. Bake until set and crisp on top, 10 to 15 minutes. Remove the stuffin' muffins to a platter and serve hot or at room temperature.
You also may use homemade croutons of any bread you choose - pumpernickel, white or whole grain work well in this recipe. Fresh herbs may also be substituted - 1/4 cup chopped parsley, thyme and sage.