Put the stuffing cubes in a large bowl and set aside. Butter a 3-quart casserole dish.
Melt 2 tablespoons of butter in a large skillet over medium-high heat. Add the sausage and break it up with a wooden spoon. Cook until it loses most of its pink color, but not so much that it's dry, about 5 minutes. Add the sausage and pan drippings to the stuffing cubes. Melt the remaining butter in the pan. Add the onion, apple, celery, and 1/2 teaspoon salt. Cook until the vegetables get soft, about 5 minutes. Add the broth and parsley and bring to a boil.
Pour the vegetable mixture over the stuffing cubes and toss until evenly moistened. Mix in the walnuts and eggs. Loosely pack the dressing in the prepared pan and cook uncovered until the top forms a crust, about 40 minutes. Drizzle about 2 tablespoons of turkey pan drippings or melted butter over the top. Cook until the top is crisp and golden, about 20 minutes more.
Put the dressing in the oven during the last hour of cooking the turkey.
To toast nuts, spread them out on a baking sheet and toast in a 350 degrees F oven until golden, about 7 minutes.