Wash the leek and cut into thick slices. Peel the onion and cut into small cubes. Peel the raw potatoes and cut into thin slices.
Dice the bacon and fry briefly. Add the onion and fry for a few more minutes. Add the leek and cook for about 5 minutes. Pour in the broth (or 3/4 l water and 1 1/2 soup cubes), stir, add potato slices and crushed garlic.
Season with thyme, nutmeg, pepper and salt and simmer covered for approx. 25 minutes after boiling.
Mix yogurt, parsley, lemon juice and egg yolk well and stir into the soup. Season with salt and pepper and serve as soon as possible.