To make the rosti, preheat oven to 180°C.
Peel and grate potatoes and onions with a coarse grater, or use a food processor attachment. Turn out onto a large clean tea towel. Squeeze as much liquid out of the mixture as you can, over a sink.
Place dry potato, onion mix into a bowl and add all other ingredients apart from oil. Stir vigorously to combine completely.
Meanwhile, heat enough oil to cover a small cast iron fry pan to 1 cm depth on a medium high heat. Dollop mixture into the hot oil and press flat to spread into a small pancake size.
Fry gently until golden, turning and repeating on the other side. Transfer to an oven tray and keep hot until you are ready to serve.
To make the Simple Hollandaise, Place all ingredients except melted butter into a food processor and blend at least 60 sec to emulsify. Scrape down sides of bowl and add melted butter through the lid slowly, as you continue to blend until a thick consistency is achieved.
Serve Rosti piping hot, with poached eggs, wilted spinach and drizzled with the Simple Hollandaise.