Preheat the oven to 350 degrees. Place the bread in a single layer on a sheet pan and bake for 20 minutes, tossing once, until lightly browned. Set aside.
Meanwhile, heat the butter in a large (10-inch) sauté pan over medium-low heat. Add the pancetta, raise the heat to medium, and cook for 5 minutes, until browned. Stir in the onions, celery, and apple and cook over medium to medium-high heat for 8 to 10 minutes, stirring occasionally, until the vegetables are tender. Stir in the sherry, rosemary, 1½ teaspoons salt, and ¾ teaspoon pepper and cook over medium heat for 5 minutes, until most of the liquid is evaporated. Off the heat, stir in the parsley.
Meanwhile, whisk the eggs, cream, chicken stock, and ¾ cups of the Gruyère in a very large bowl. Stir in the bread and the vegetable mixture and set aside for 30 minutes to allow the bread to soak up the custard. Pour into a 9 × 9 × 2-inch oven-to-table baking dish. Sprinkle with the remaining ¼ cup of Gruyère and bake for 45 to 50 minutes, until the top is browned and a knife inserted into the middle comes out clean. Serve hot.
Make Ahead: Assemble early in the day, cover with plastic wrap, and refrigerate. Bake before serving.
Copyright 2016, Cooking for Jeffrey, Clarkson Potter/Publishers, All Rights Reserved