Preheat the oven to 350 degrees F. Spread the bread cubes in a single layer on a sheet pan and bake for 20 minutes, until lightly browned. Set aside.
Meanwhile, heat the butter in a large (12-inch) saute pan over medium-low heat. Add the pancetta, raise the heat to medium, and cook for 5 minutes, until starting to brown. Stir in the onion, celery, and apple and cook over medium heat for 8 to 10 minutes, stirring occasionally, until the vegetables are tender. Stir in the sherry, rosemary, 1 tablespoon salt, and 1 1/2 teaspoons pepper and cook for 5 minutes, until most of the liquid is gone. Off the heat, stir in the parsley.
Meanwhile, beat the eggs, cream, chicken stock and 1 1/2 cups of the cheese together in a very large mixing bowl. Add the bread cubes first and then the vegetable mixture, stirring well to combine. Set aside at room temperature for 30 minutes to allow the bread to soak. Stir well and pour into a 2 1/2- to 3-cup gratin dish (13 x 9 x 2 inches). Sprinkle with the remaining 1/2 cup of Gruyere and bake for 45 to 50 minutes, until the top is browned and the custard is set. (Insert a knife in the middle to check.) Serve hot or warm.