Preheat the oven to 350 degrees F. Spray a 9 by 13-inch baking dish with nonstick cooking spray. Place the raisins in a small bowl and cover with 1 cup hot water. Set aside.
Heat the oil in a large skillet over medium heat. Add the onions and celery, season with salt, pepper, and poultry seasoning, and cook until softened, 6 to 7 minutes.
Drain the raisins, reserving the liquid, and put them into a large bowl with the bread, walnuts, parsley, and sage. Add the cooked onions and celery. Mix everything together, pour over the chicken broth, season with salt and pepper, and mix again. If the mixture seems too dry, add some of the reserved soaking liquid from the raisins. Spoon the stuffing into the prepared baking dish (stuffing can be made ahead to this point, covered, and refrigerated overnight). Bake until the top is golden brown, about 30 minutes.