Chop onion and garlic. Strain lentils and collect lentil water. Fry onion in oil until lightly browned.
Sauté the garlic and make a roux with the flour. Pour in the lentil water and add the bouillon cube.
Add the bay leaf, marjoram and lentils and simmer gently, covered, for 25 minutes.
At the end of the cooking time, season with vinegar, salt and pepper and serve.