Clean and trim the vegetables. Peel and small-dice the carrots and celery, finely chop the onion or leek, and cut the potatoes into medium dice.

Place the dried lentils in a fine-mesh strainer and rinse under cold water until the water runs clear; set aside.

Heat the butter (or oil) in a large stockpot over medium heat. Add the chopped onions, carrots, celery, and potatoes and cook for about 5 minutes, allowing them to soften without excessive stirring.

Add the rinsed lentils, vegetable broth, and bay leaf to the pot. Bring the mixture to a boil, then reduce the heat, cover, and let it simmer gently for about 30 minutes until lentils and potatoes are tender.

Stir in the red wine vinegar, tomato paste, and sugar. Taste and adjust with salt and freshly ground pepper as needed to balance the flavors.

If using Wiener sausages, either slice and stir them into the soup to heat through, or warm whole sausages separately in hot water (do not boil or the casing may split) before serving.

Ladle the soup into bowls, garnish with chopped parsley, and serve with hearty bread such as rye sourdough. Enjoy warm.