1 stick unsalted butter, plus more for the baking dish
1 pound sweet Italian sausage, casings removed
4 stalks celery, chopped
2 onions, chopped
2 tablespoons chopped fresh sage
3 cups low-sodium turkey or chicken broth
Kosher salt and freshly ground pepper
2 large eggs
1/2 cup chopped fresh parsley
16 cups 1/2-inch stale country white bread cubes (1 1/2 pounds)