16-ounce bag stuffing cubes
6 tablespoons unsalted butter, plus more for greasing the pan and topping
1 pound fresh sage sausage, casing removed
1 medium onion, chopped
2 cooking apples, such as Gravenstein, Rome, or Golden Delicious, peeled, cored, and chopped
1 to 2 ribs celery with leaves, chopped
1/2 teaspoon kosher salt
3 cups chicken broth, homemade or low-sodium canned
1/4 cup chopped fresh flat-leaf parsley
1/2 cup walnut pieces, toasted (See Note)
2 eggs, beaten