Preparation

Prep the bread and vegetables

• Position a rack in the center of the oven and heat the oven to 350°F. Toast the bread cubes on a rimmed baking sheet in the oven until crisp on the outside, about 15 minutes. Cool on a rack to room temperature.
• Melt the butter in a 12-inch skillet over medium heat. Add the leeks, apples, 1 tsp. salt, and a pinch of pepper. Toss, cover, and cook, stirring occasionally, until the leeks are beginning to soften, about 8 minutes. Uncover, reduce the heat to medium low, and cook, stirring occasionally and adding a tablespoon of water at a time if the pan looks dry, until the leeks are very tender and slightly golden, about 10 minutes more.
• Add the celery and sage, and cook, stirring occasionally, until the celery has softened slightly, 2 to 3 minutes. Remove from the heat and let cool to room temperature.

Assemble and bake the stuffing

• Position a rack in the center of the oven and heat the oven to 375°F.
• In a large bowl, toss the bread cubes with the leek mixture, prosciutto, parsley, and lemon zest. Season generously with salt and pepper. Drizzle 1/2 cup of the chicken broth over the stuffing, toss, and let sit 5 to 10 minutes. The mixture should be moist but not soggy. If it still seems dry, add up to 1/2 cup more broth. Transfer the stuffing to a 9×13-inch baking dish, dot the top with the butter, and cover with foil. Bake until heated through, 20 to 30 minutes. Remove the foil, and bake until the top is browned, about 15 minutes more. Let cool slightly before serving.

Make Ahead Tips

The bread cubes can be toasted up to 2 weeks ahead. Once cooled, seal in a zip-top freezer bag and freeze. Thaw overnight at room temperature before continuing.

The leeks, apples and celery can also be sautéed up to 2 weeks ahead. Once cooled, seal in a zip-top freezer bag and freeze. Thaw overnight in the refrigerator, then remove from the fridge and let sit at room temperature for 2 hours before assembling the stuffing.

The prosciutto can be chopped, sealed in a zip-top freezer bag, and frozen for up to 2 weeks. Thaw overnight in the refrigerator before continuing.