Roughly chop the potatoes and peel, quarter and core the apple. Cook in simmering salted water for 20 mins. Drain well and mash with the butter, salt and pepper.
Heat the oven to 200C/390F/Gas 6. Mash the black pudding with a fork and gently fry in a pan for 2 minutes, turning, until crisped.
Place an oiled ring mould (8 cm wide, 6 cm high) on a baking tray lined with greaseproof paper, and arrange a layer of black pudding at the base, pressing down gently.
Top with a layer of mash, then repeat, finishing with a layer of potato.
Gently remove the mould and repeat until you have four cakes.
Scatter with cheese and bake for 15mins to heat through (finish under the grill if not golden brown).
Gently heat the cream, mustard, salt and pepper, stirring.
Transfer the cakes to warm plates, top with the salad greens, and serve with the mustard sauce