8 cups (¾-inch-diced) white bread cubes, crusts removed
4 tablespoons ½ stick) unsalted butter
2 cups chopped yellow onions (2 onions)
1 ½ cups medium-diced celery
1 Granny Smith apple, peeled and chopped
½ cup medium or dry sherry
Kosher salt and freshly ground black pepper
½ cup chopped fresh parsley leaves
7 extra-large eggs
2½ cups heavy cream
1¼ cups chicken stock, preferably homemade
2 cups grated Gruyère cheese, lightly packed (6 ounces with rind), divided
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 (12-inch) sauté pan over medium-low heat. 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, 1 tablespoon salt, and 1½ teaspoons pepper and cook over medium heat for 5 minutes, until most of the liquid is gone. Off the heat, stir in the parsley.
Meanwhile, whisk the eggs, cream, chicken stock, and 1½ 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 × 13 × 2-inch oven-to-table baking dish. Sprinkle with the ½ 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.
Assemble early in the day, cover with plastic wrap, and refrigerate. Bake just before serving