How to Make It
Step 11. Preheat oven to 350°F.
Step 22. Place bread in a single layer on a sheet pan; bake for 20 minutes, tossing once, until lightly browned. Set aside.
Step 33. Meanwhile, heat butter in a large sauté pan over medium-low heat. Add pancetta, raise heat to medium, and cook for 5 minutes, until browned. Stir in onions, celery, and apple; cook over medium heat for 8 to 10 minutes, stirring occasionally, until vegetables are tender. Stir in sherry, rosemary, 1 teaspoon salt, and 1½ teaspoons pepper; cook for 5 minutes, until most of the liquid is gone. Off the heat, stir in parsley.
Step 44. Whisk eggs, heavy cream, chicken stock, and 1½ cups of the cheese in a very large bowl. Stir in bread and vegetables; set aside for 30 minutes so bread can soak up the custard.
Step 55. Pour into a 9-by-13-by-2-inch oven-to-table baking dish. Sprinkle with remaining Gruyère; bake for 50 to 60 minutes, until the top is browned and a knife inserted into the middle comes out clean. Serve hot.