Remove giblets and neck from duckling; reserve for other uses. Rinse duckling thoroughly with cold water, and pat dry. Prick fatty areas of duckling with a fork at intervals. (Do not prick breast.) Lift wingtips up and over back, tucking under duckling securely. Combine 1/2 teaspoon salt and 1/4 teaspoon pepper; sprinkle over surface and in cavity of duckling. Set duckling aside.
Sauté onion in butter in a large skillet until tender. Add next 5 ingredients for dressing, mixing well. Spoon dressing into cavity of duckling. Shape remaining dressing into 1 1/2- inch balls; cover, and refrigerate. Close cavity of duckling with skewers; truss.
Fold neck skin under and place duckling, breast side up, on rack in a roasting pan. Bake, uncovered, at 325° for 2 1/2 hours or until drumsticks and thighs are easy to move. During last 30 minutes of cooking time, place stuffing balls on rack in roasting pan. When done, transfer duckling to serving platter. Arrange stuffing balls around duckling; garnish with celery leaves and apple wedges, if desired.