Boil potatoes and apple in salted water for 20 minutes.
Drain and mash potatoes and apple with butter, salt, and pepper.
Preheat the oven to 200°C.
Mash the black pudding and fry in a hot pan for two minutes until crisp.
Place an oiled 8cm ring mould on a lined baking tray.
Layer black pudding at the base followed by mash, repeating once to finish with potato.
Remove the mould and repeat to create four separate cakes.
Scatter cheddar over the cakes and bake for 15 minutes.
Grill briefly if the tops are not yet golden brown.
Heat cream and mustard in a pan with seasoning, stirring frequently.
Serve cakes on warm plates topped with salad greens and mustard sauce.