Preheat the oven to 350°F and grease a 9 x 12 baking dish.
Place bread on rimmed baking sheet and toast until golden brown, about 15 to 20 minutes. Once complete, transfer to large bowl. (You can also skip this step by cutting the bread and leaving it to dry overnight.)
In a large pan, heat the butter on medium heat. Add the leeks, celery, and garlic and cook, stirring occasionally, until very soft, about 10 minutes.
Add apple, sage, thyme, salt and pepper and cook for an additional 2-3 minutes until fragrant. Once cooked, transfer to the bowl with the bread.
In the same pan, add sausage, casings removed, and cook over medium heat for about 10 minutes, until browned and cooked through, breaking up the sausage with a fork while cooking. Once cooked, transfer sausage to the bowl.
Add the whisked egg and chicken broth to the bread and sausage mixture, stirring to ensure that everything is well combined.
Transfer the stuffing mixture to the greased baking dish and transfer to the oven. (If you prefer to cook it directly in the turkey, add it to the cavity at this point.)
Bake covered for 20 minutes, and then uncovered for an additional 10 minutes until brown on top and warm in the middle.
Allow to cool slightly and sprinkle with parsley to serve.