Melt the butter in a small pan and press the potato onto the base of the pan.
Cook the potato until golden brown on one side, then flip it over with a palette knife and cook on the other side. Season and keep warm.
Place the spinach in a small saucepan and heat gently until it just starts to wilt, then season.
To make the hollandaise, place the egg yolks and white wine vinegar in a small bowl and whisk over a pan of hot water until thickened. Gradually add the butter by pouring it and whisking it into the yolk mixture.
Pour and whisk, working quickly, until all the butter is used up. Season to taste.
Preheat the grill to high. Place the rosti (potato) on a heatproof serving plate and the spinach on top.
Then add the smoked salmon, cover with the hollandaise and sprinkle with Parmesan cheese.
Place under the grill until the hollandaise is light brown. Sprinkle the chives on top, cut into wedges and serve.