1. 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.
2. Meanwhile, heat the butter in a large (12-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, 1 tablespoon salt, and 1 1/2 teaspoons pepper and cook over medium heat for 5 minutes, until most of the liquid is gone. Off the heat, stir in the parsley.
3. Meanwhile, whisk the eggs, cream, chicken stock, and 1 1/2 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 x 13 x 2-inch oven-to-table baking dish. Sprinkle with the 1/2 cup of Gruyère and bake for 50 to 60 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 just before serving.

Reprinted from Cooking for Jeffrey [2]. Copyright © 2016 by Ina Garten. Photographs by Quentin Bacon. Published by Clarkson Potter/Publishers, an imprint of Penguin Random House, LLC.