Preheat the oven to 200°C (400°F).

In a large bowl, combine the grated potatoes, grated onion, salt, and pepper. Mix well.

Heat a non-stick skillet over medium heat and add a little oil. Take a quarter of the potato mixture and press it into the skillet to form a rosti. Cook until golden brown and crispy on both sides. Repeat for the remaining rostis.

In a saucepan, bring water to a simmer. Add a splash of vinegar. Crack an egg into a small bowl and gently slide it into the simmering water. Poach for about 3-4 minutes or until the whites are set but the yolk is still runny. Remove with a slotted spoon and drain on a paper towel. Repeat for the remaining eggs.

In a heatproof bowl, whisk together the egg yolks and lemon juice. Place the bowl over a pot of simmering water (double boiler), making sure the bottom of the bowl doesn't touch the water. Whisk constantly and slowly drizzle in the melted butter until the sauce thickens. Season with salt and pepper.

To assemble, place a potato rosti on each plate. Top with flaked hot smoked salmon, a poached egg, and drizzle with hollandaise sauce. Garnish with chopped fresh parsley.

Serve immediately and enjoy!