Meanwhile, heat the butter in a large (12-inch) saute pan over medium-low heat. Add the pancetta, raise the heat to medium, and cook for 5 minutes, until starting to brown. Stir in the onion, celery, and apple and cook over medium heat for 8 to 10 minutes, stirring occasionally, until the vegetables are tender. Stir in the sherry, rosemary, 1 tablespoon salt, and 1 1/2 teaspoons pepper and cook for 5 minutes, until most of the liquid is gone. Off the heat, stir in the parsley.