1 (16-ounce) loaf sourdough bread
5 tablespoons unsalted butter
2 medium yellow onions
1/2 medium fennel bulb (about 8 ounces)
2 stalks celery (about 6 ounces total)
2 cloves garlic
2 medium apples, such as gala, honeycrisp or pink lady (about 1 pound total)
1 small bunch fresh sage
1 small bunch fresh thyme
1 small bunch fresh parsley
1 pound uncooked pork sausage, preferably bulk
2 1/4 teaspoons kosher salt, divided, plus more as needed
1/4 cup dry white wine (optional – see Recipe Note)
1 1/2 cups low-sodium chicken broth
1 large egg
Arrange 2 racks to divide the oven into thirds and heat the oven and heat the oven to 300°F.
Tear or cut 1 loaf sourdough bread into 1/2-inch pieces (about 14 cups). Transfer the bread onto 2 rimmed baking sheets and spread into an even layer.
Bake until the edges of the bread are dry, about 20 minutes. Meanwhile, cut 5 tablespoons unsalted butter into 5 pieces. Cut 1 piece in half; use one of the pieces to coat a 9×13-inch baking dish, then use the second piece to coat a sheet of aluminum foil large enough to cover the baking dish.
When the bread is ready, set aside to cool. Heat the oven to 425°F.
Prepare the following, placing each in the same medium bowl as you complete it: Finely chop 2 medium yellow onions (about 3 cups), 1/2 medium fennel bulb (about 1 1/2 cups), and 2 large celery stalks (about 1 cup). Mince 2 garlic cloves.
Prepare the following, placing each in the same second medium bowl as you complete it: Finely chop 2 medium apples (about 3 cups). Pick 2 tablespoons fresh thyme leaves and finely chop. Pick and finely chop fresh sage leaves until you have 1/4 cup.
Finely chop fresh parsley leaves until you have 2 tablespoons and leave on the cutting board.
Heat 1 piece of the unsalted butter in a large skillet over medium-high heat until shimmering. Add 1 pound pork sausage (remove casings first if needed) and 1/4 teaspoon of the kosher salt. Cook, breaking up the sausage with a wooden spoon into bite-size pieces, until browned and just cooked through, 6 to 8 minutes. Transfer with a slotted spoon to the largest mixing bowl you have (you can also use a large pot).
Reduce the heat to medium and add 1 piece of the unsalted butter to the skillet. Once melted, add the onion mixture and 2 teaspoons of the kosher salt. Cook, stirring occasionally, until the vegetables are tender, about 10 minutes. Add 1/4 cup dry white wine if using and cook until the liquid is evaporated, about 3 minutes. Transfer to the bowl with the sausage.
Add the remaining 2 pieces unsalted butter to the skillet. Once melted, add the apple mixture and cook until the apples are beginning to soften, about 4 minutes. Transfer to the bowl with sausage and vegetables.
Add the parsley and dried bread to the bowl and gently toss until everything is evenly combined. Taste and season with more kosher salt as needed.
Whisk 1 1/2 cups low-sodium chicken broth and 1 large egg together in one of the now-empty medium bowls until well blended. Pour the egg mixture over the bread mixture and toss until everything is evenly moistened.
Transfer the stuffing to the baking dish and spread into an even layer. Cover the baking dish with the foil buttered-side down.
Bake on the upper rack for 45 minutes. Uncover and bake until the top is browned and crisp, about 15 minutes more. Let cool 10 to 15 minutes before serving.