Peel the potatoes and cut into small pieces. Clean the leek and cut into fine rings. Set aside the green rings of the leek.
Heat 2 tablespoons of Ländle butter in a saucepan and fry the white parts of the leek and potatoes in it. Deglaze with vegetable stock and simmer for about 15 minutes.
In the meantime, boil the lentils in 2.5 times the amount of water for about 30 minutes until they are soft. Grate the Vorarlberger Bergkäse.
When the potato and leek vegetables are soft, add the grated Vorarlberger Bergkäse and Ländle cream.
Puree finely with a hand blender. Season to taste with marjoram, caraway seeds and pepper, add salt if necessary.
Strain the lentils. Melt 1 tablespoon of Ländle butter in a pan and briefly fry the green leek rings. Then add these to the lentils, stir in the chopped hazelnuts and season with salt and balsamic vinegar.
Arrange the lentil mixture on the soup and enjoy.