Place a Dutch oven or a large pan over medium-high heat, then add the bacon bits.
Cook until they turn brown and crunchy on the edges --- then transfer them to a plate lined with kitchen paper to remove excess oil. Set aside.
Using the same pan and oil, add the onion and saute until it softens.
Sprinkle the flour and lower the heat to medium.
Cook until it turns brown, stirring constantly to avoid burning the flour --- this typically takes at least a couple of minutes.
Add the drained lentils and the crispy bacon from earlier. (NOTE: Remember to keep some bacon for garnish later, if that’s what you prefer.) Sauté them for a minute, then pour the stock and cover with a lid.
Bring the heat back to medium-high, then bring to a soft boil; adjust the heat to low and simmer until the lentils are tender.
Add the carrots and potatoes and continue simmering until the vegetables are cooked.
Add the chopped sausage and a teaspoon of vinegar --- let cook for a couple of minutes.
Mix, taste, and adjust seasoning by adding salt, pepper, and vinegar (if needed).
Cover with a lid and remove from the heat.
Transfer your German lentil soup into serving bowls --- enjoy!