Preheat the oven to 350°F. In a medium skillet, cook ground sausage, celery, and onion over medium heat until the sausage is browned and the vegetables are cooked through.
In a large bowl, toss together bread cubes, cooked sausage, onions, celery, and diced apple.
Combine 2 cups chicken broth, the melted butter, salt, pepper, garlic powder, onion powder, thyme, and beaten eggs. Whisk together until thoroughly combined.
Pour broth mixture over the bread cube mixture and toss to distribute the liquid evenly over the stuffing mixture. The bread mixture should be moistened throughout, but not soggy. Add additional chicken broth in small increments, as needed, until the bread is thoroughly moistened.
Transfer the stuffing mixture to a greased 9x13 casserole dish. Cover with aluminum foil and bake approximately 40 minutes until the apples and stuffing is cooked through.
For a golden brown and crispy top, remove the foil and continue cooking 15-20 minutes longer.
Serve with parsley for garnish, if desired.