To make the rosti, grate the potato into a clean tea-towel and squeeze any excess water out. Pop the potato into a bowl, add the egg yolk, season and mix well. Divide into 4 and then form into a rosti shape (flat circular shape approx. 15cm in diameter).
Place a non-stick frying pan over a medium heat add the sunflower oil. Carefully place in the rosti in the pan and cook for 4-5 minutes before turning over. The rosti should be golden and crisp before turning.
While the rosti is cooking place a large pan filled with water about 3” deep for poaching the eggs with the vinegar. This water should be barely simmering.
For the Hollandaise, melt the butter but make sure the butter is not hot. Place the eggs and vinegar into a bowl and while whisking slowly. Pour in the melted butter a little at a time. Leave the white sediment in the jug. Add a squeeze of lemon juice and a tsp of hot water and leave until needed.
Cook the asparagus in a small saucepan of water for 3 minutes.
Crack the eggs into a large pan of simmering water with the vinegar and poach for 3-4 minutes for a runny yolk.
To assemble place the golden crisp rosti onto your plate, add the asparagus, followed by the poached egg and finally drizzle over the hollandaise sauce.