Heat the water in a deep saucepan without boiling and bain-marie the egg yolks.
To make plain butter, put the butter in a small saucepan. Melt the butter over low heat without burning it and without letting it foam too much. Remove the milk foam with the help of a spoon.
Strain the melted butter through a sieve and keep it aside. Put the carefully separated egg yolks, white wine vinegar, and cold water in a glass bowl. Mix with an awhisk.
Start and stir the egg yolks in the same direction at all times. Place the bowl over hot water at a temperature of around 50 degrees and continue whisking. After 2-3 minutes, when the egg yolks have thickened to a creamy consistency, place the bowl on the kitchen counter.
Let the whipped egg yolks stand for a short while to cool down, then pour the plain butter that you have obtained into the egg yolks slowly and continue stirring non-stop for this. When the sauce turns into a fine-textured cream, add finely ground sea salt and freshly ground black pepper if you wish.
Add the vinegar and salt to the slightly boiling water, stir the water in a clockwise motion with a spoon to create a whirlpool, and then carefully drop the eggs into the center one by one. After 4 minutes, place the cooked eggs on a paper towel using a colander. Toast the sourdough bread in the oven, brushing it lightly with butter. Saute the spinach in a hot pan, add the zest of half a lemon, and season with salt.
Serve the toasted sourdough bread, sauteed spinach, poached eggs, and Hollandaise sauce on top of each other.