Spray a 9x13 baking dish (or any 3 quart casserole dish) with nonstick cooking spray.
Heat a large skillet over medium heat. Add butter and when butter is melted, add celery, onion, leeks, apple, sage, parsley, and ½ teaspoon salt. Cook, stirring frequently, until celery and onion are soft, 8 to 10 minutes.
Add sausage and cook, breaking it apart, until cooked all the way through, 8 to 10 minutes more.
Transfer sausage and vegetables to a large mixing bowl. Let cool slightly.
In a medium mixing bowl, whisk together eggs and ½ teaspoon salt until smooth. Slowly add in stock while whisking so that everything is evenly combined.
Pour stock mixture over sausage and vegetables. Add bread cubes on top.
Gently fold everything together until most of the stock mixture has been absorbed by the bread cubes (this may take a few minutes of stirring / folding).
Transfer stuffing to prepared baking dish. Cover tightly with foil.
Refrigerate overnight (or for about 8 hours).
When ready to bake, let stuffing come to room temperature.
Preheat oven to 350°F / 177°C.
Bake stuffing, covered, for 40 minutes.
Remove foil and continue baking until stuffing is golden brown on top, about 30 minutes more.