The combination of vinegar and just a touch of sugar gives it its famous sweet-sour flavor balance. Coupled with smoky bacon and flavorful veggies, it's served over Spätzle and often with a sausage for a favorite comfort dish throughout southern Germany.
Cook the bacon over medium-high heat until done (for best flavor don't drain the bacon grease). Add the onions and cook until soft and translucent 5-7 minutes. Add the butter, carrots and leek and cook for 5 minutes.Add the lentils, broth, bay leaf, salt, pepper and sugar and bring to a boil. Reduce the heat to medium-low, cover and simmer for 40 minutes. Add the vinegar and parsley and simmer another 3-4 minutes. If too thick for your taste, add a little extra beef broth. Add more salt, pepper, sugar and vinegar to taste.Serve over Spätzle, commonly with a German Saitenwurst or a sausage of your choice.