1. Peel the potatoes and boil in lightly salted water until cooked; drain and mash well. Heat the milk, 100g butter and all but 2 tbsp spring onions until boiling, and cook for 1 minute. Beat the mixture into the potatoes with the mustard, seasoning with salt and pepper to taste.
2. If using whole small black puddings, cut in half. Heat half the remaining butter in a frying pan and cook the black puddings for 5 minutes on each side. Set aside and keep warm.
3. Add the remaining butter to the pan and fry the apple slices until golden brown on each side. Set aside.
4. Add the remaining spring onions to the pan and cook for 2-3 minutes. Raise the heat and add the cider and sage. Bubble briskly until reduced by half. Add the cream and season. Bubble for 1-2 minutes then return the apple slices to the pan to reheat.
5. Reheat the mashed potato if necessary and serve with the black pudding, apple slices and cider sauce. Serve immediately.