1. Preheat oven to 400° with racks in upper and lower thirds. Spray 3-quart baking dish with cooking spray.
2. Put dried porcinis in a heatproof bowl and pour boiling water over them. Let steep 30 minutes. Pour mushrooms with liquid through a paper towel-lined sieve into a bowl, pressing to extract liquid. Reserve 1 cup mushroom water. Transfer porcinis to a cutting board and chop.
3. Arrange bread in one layer in 2 large, shallow baking pans and toast, switching position of pans halfway through baking, until golden and dry (10-15 minutes). Cool; transfer to large bowl.
4. Toast hazelnuts on a baking pan in oven until fragrant (6-8 minutes). Transfer to a kitchen towel and rub off loose skins. Let cool and coarsely chop.
5. Meanwhile, cook shallots in butter and oil in a 12-inch heavy skillet over medium heat, stirring occasionally, until softened and golden (about 6 minutes). Add porcinis and wild mushrooms, celery, sage, and 1/2 teaspoon salt, and cook, stirring occasionally, until liquid from mushrooms has evaporated (20-30 minutes). Add sherry and deglaze skillet by boiling, stirring, and scraping up any brown bits, until sherry is reduced by half (about 2 minutes). Add vegetable mixture to bread in bowl. Add hazelnuts and parsley and toss.
6. Whisk together chicken broth and reserved mushroom water, eggs, remaining 1/2 teaspoon salt, and pepper; stir into bread mixture. Transfer to baking dish. Bake, loosely covered with a sheet of foil, in lower third of oven (30 minutes). Remove foil and bake until top is browned (about 20 minutes more).