Heat the butter in a saucepan. Add the whole wheat flour and sweat a little. Add 250 ml spring water and stir with a whisk until a creamy soup is formed. Then add another 250 ml of spring water while stirring. Simmer gently for 10 minutes, stirring occasionally.
Trim and wash the leek and cut off the dark green leek tips. Quarter the remaining light-colored leek sticks lengthways and cut into strips, place in a colander and rinse under running water. Shake out and add to the soup. Cook for 5 minutes.
Mix the egg yolks with the sour cream in a small bowl and add to the no longer boiling soup. Stir everything again and season with Himalayan salt, granulated broth and nutmeg. Let the soup swell briefly in the residual heat.