Granny Smith apples and acorn squash bring an earthy sweetness to this tangy sourdough stuffing/dressing, while hazelnuts add a satisfying buttery crunch.
Preheat oven to 300 degrees. Butter a 2 1/2- to 3-quart baking dish. Spread bread in a single layer on a rimmed baking sheet and bake, stirring once halfway through, until completely dry and turning golden in places, 45 to 55 minutes. Transfer to a large heatproof bowl.
Increase oven temperature to 375 degrees. Spread hazelnuts on a rimmed baking sheet; toast 12 to 15 minutes. Remove; when cool enough to handle, coarsely chop. Meanwhile, melt butter in a large skillet over medium-high heat. Add onion, celery, squash, apples, and 2 teaspoons salt; cook, stirring occasionally, until vegetables and fruit are crisp-tender but have not developed color, about 10 minutes. Add wine; cook until mostly evaporated, about 2 minutes. Transfer mixture to bowl with bread. Stir in chopped parsley and stock to combine. Stir in eggs until bread mixture is evenly coated, then transfer to prepared dish. Top with buttered parchment; loosely cover with foil.
Bake 20 minutes. Uncover and continue baking until golden and crisp on top, 20 to 25 minutes more. Sprinkle with hazelnuts and parsley leaves; serve.