Rinse lentils very well with cold water- pick them over and remove any nasty bits or pieces of stone.
You can soak them overnight in cold water if you like, then drain before using.
In a large stockpot, place drained lentils, 6 cups fresh cold water, bouillon cubes, and just half of the diced onion.
Cook for 1 1/2 hours on medium-low heat, covered.
Add the potatoes and cook until they are tender- 30-40 minutes approximately.
In a small pan, melt the butter and saute the remaining onions until browned.
Add the flour to the onions and stir until combined.
Add to the soup, stirring constantly to combine.
Add the vinegar and season with salt& pepper to taste.
Optional:If you would like to add some meat to the soup, simply have pre-cooked sausages (you can even use German frankfurters if desired) sliced into small rounds and drop them into the pot during the last 15 minutes of cooking time.