Cut the soup vegetables into small pieces and cook them with the lentils in 2 liters of stock until soft.
Melt the butter in a saucepan, dust with flour and make a roux. Add hot lentil broth with a ladle and then add to the soup.
Cut the bacon and onions into cubes, sauté and add to the soup. Let everything simmer for 10 minutes, season with salt and pepper. If you want, you can add the vinegar or put it on the table to season.