For the beetroot and potato rosti, preheat the oven to 200C/400F/Gas 6.
Place the grated potatoes into a clean tea towel inside a bowl and squeeze out any excess water into the bowl.
Transfer the grated potato and grated beetroot into a clean bowl along with the crème fraîche, egg, parsley, salt and pepper. Stir until well combined.
Heat the butter in an ovenproof frying pan over a medium heat and place 2 x 7.5cm/3in chefs' rings in the pan. Spoon the potato mixture into the rings, press down well using a spoon, and cook for 3-4 minutes, or until golden-brown. Remove the rings, turn the rosti over, press the mixture down and cook for 3-4 minutes on the other side.
Transfer the pan to the oven and bake the rosti for 5-10 minutes, or until tender and cooked through.
For the hollandaise sauce, put the egg yolk, lemon juice and a tablespoon of water in a blender. Turn on the blender and slowly pour the warmed butter through the lid, the sauce will start to thicken. Season with cayenne pepper and salt.
For the corned beef and egg, heat a large frying pan and add the butter, once hot, add the sliced corned beef. Cook on each side for 2-3 minutes, until crisp and golden-brown. Remove from the pan.
Add the oil to the pan. Break the eggs into the pan and fry until cooked to your liking.
To serve, place the rosti in the centre of serving plates and place the fried corned beef on top. Top with the fried egg and pour over the hollandaise sauce.