Blitzing the onion and celery in a food processor—versus chopping—disperses both flavor and moisture throughout the dish. Fresh cranberries add juicy pops of tangy flavor, while the dried fruit adds chewy bits of sweetness.
Preheat oven to 275°F. Spread bread on a large rimmed baking sheet. Bake until dry to the touch, about 30 minutes. Let cool; transfer to a large bowl.
Meanwhile, pulse onion and celery in a food processor until finely chopped. Cook sausage in a large skillet over medium heat, crumbling with a wooden spoon, until browned, about 5 minutes. Leaving the fat in the skillet, transfer the sausage to a plate. Add the onion and celery to the pan and cook, stirring occasionally, until softened and lightly browned, about 6 minutes. Add the sausage, apple and garlic and cook for 1 minute. Let cool for 5 minutes.
Increase oven temperature to 350°. Coat a 9-by-13-inch baking dish with cooking spray and coat one side of a piece of foil with cooking spray.
Add the sausage mixture, eggs, fresh cranberries, walnuts, parsley, dried cranberries, sage, pepper and salt to the bread; mix well. Slowly add broth, 1/2 cup at a time, allowing it to absorb before adding more. For a crispier stuffing, add about 3 cups total; if you like it moister, add more broth. Transfer the stuffing to the prepared baking dish, pressing down on it. Drizzle with oil. Cover with the foil, sprayed-side down.
Bake until the internal temperature registers 165°F, about 50 minutes for moist stuffing. For crispier results, cover and bake for 30 minutes, then uncover and bake until the top is lightly browned and crisp, 20 to 30 minutes more.