Preheat the oven to 200°C.
Squeeze any excess water out of the grated potatoes using a clean tea towel.
Mix the potato with crème fraîche, egg yolk, salt, and pepper in a bowl.
Heat 75g of butter in an ovenproof frying pan and place two chefs' rings in the pan.
Spoon the potato mixture into the rings and cook for 4 minutes per side until golden brown.
Bake the rosti in the oven for 10 minutes until tender.
Fry the black pudding slices in oil until golden-brown and cooked through.
Sauté apple wedges in 25g of butter and simmer with cider until the liquid evaporates.
Serve the rosti topped with black pudding and apples.