Take large pot and preheat the oil on medium heat.
Peel & chop the onions. Fry it with a bit of salt until golden brown.
In the meantime chop smoked bacon, add to to pot. Stir.
Dice: carrots, celery tuber, potatoes and parsley root. Add them to the pot and stir.
Add the lentils and cook for 20-30 minutes (depending in the lentils you use).
Heat up & slice Frankfurt sausage.
After the lentils and vegetables are tender season the soup with vinegar, sugar, salt and pepper.
Enjoy it hot with Frankfurt sausage!