Grate the potatoes and onion and toss in 1 ts salt and set aside for 20minutes.
Squeeze out excess and combine with flour dill and chives.
Divide into 8 messy “patties” and fry over medium heat in a large pan in vegetable oil until golden pan both sides, about 4 minutes a side. Keep warm until serving.
In a jar that can accomodate a stick blender, melt the butter, and while still warm add the lemon, mustard, tarragon and the egg yolks, then blend with the stick blender until completely emulsified. Set aside until ready to serve.
Bring a deep pan (non stick is helpful!) of water to a soft simmer with the 1 tbs of white vinegar. Don’t be afraid here… the best way to approach the many poach is to just crack them all in and let them softly cook. Gently loosen and lift the whites incase they are sticking. It’s a good time to turn the water off and dish up everything else while they finish cooking.
Arrange each plate with 2 warm röstis and some curls of cold smoked salmon. Top with perfectly poached eggs and a decent pour of hollandaise, serve with some baby spinach or salad leaves and lots of salt and pepper.