Add the boiled potatoes, egg yolks, butter, cream, and spices to a bowl and mash the potatoes with a fork. Whip the entire mixture with the fork to make the sauce as smooth as possible. Then stir in the whipped egg whites and pour the potato sauce into a piping bag.
Pipe the potato cream in strips on a parchment paper-lined cookie sheet pan. The pattern should be similar to the picture below.
Melt the butter in a pan and sweat the onion and garlic in it. Add the spinach leaves and let them cook down before seasoning with salt, pepper, and nutmeg. Pour the cooked spinach into a tall bowl and add the cream and egg yolks. Purée the mixture with a hand blender, add the whipped egg whites, and sift the flour. Stir everything until a uniform cream is formed.
Spread the spinach cream over the potato pattern and smooth it out with a spatula. Leave a narrow edge so that the ends of the potato pattern will slightly extend beyond the spinach. Cook for 20 minutes at 390°F in a preheated oven.
After removing the pan from the oven, flip the potato and spinach sheet onto a clean kitchen towel and remove the parchment paper. Dip another clean cloth in cold water, wring it out, and place it over the potato and spinach sheet and let it cool for 30 minutes. Then flip the potato and spinach sheet over once again.
Smear the cream cheese over the spinach and smooth it out, again leaving a narrow edge. Layer the smoked salmon slices on top. Using the kitchen towel, roll up the salmon spinach potato log and let it cool before serving.
