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 hot Homemade German Spätzle with a sausage (preferably German Saitenwurst if you can find it).
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