Cut the carrot, celery and leek into tiny cubes. Cut the potatoes into cubes with an edge length not exceeding 5 mm.
Wash the lentils, drain them and simmer with the vegetable stock for 30 minutes.
Melt the butter in a second saucepan and briefly sweat the vegetables with the potatoes. Deglaze with the beef stock and simmer for 5 minutes. Add the lentils and mix everything well. Season to taste with vinegar.
Finely chop parsley or chives and garnish the filled plates with them.
This is a nice variant for the beginning of the week if you have eaten a lot of meat on the weekend.
The soup becomes more hearty, but also richer in calories, by adding finely chopped pork belly or smoked meatballs.