Prep Stuffing: Combine celery, apples, walnuts, breadcrumbs, sausagemeat, egg, and parsley in a bowl with a tablespoon of mustard and seasoning. Mix well.
Season Chicken: Thoroughly season the chicken and place it in the large roasting tin. Coat with some butter.
Roast Chicken: Roast in a preheated oven at 190C for 50 minutes.
Prepare Stuffing Tin: Grease a smaller tin and add the stuffing mixture, sprinkle with reserved breadcrumbs and dot with remaining butter.
Glaze and Continue Roasting: Brush the chicken with remaining mustard, move to a lower shelf in the oven and roast for an additional 40 minutes alongside the stuffing until both are golden.
Rest and Serve: Allow the chicken to rest for 10 minutes before serving with the golden crisp stuffing.