Hungarian Lentil Soup and Stew

For stew, chop onion into large pieces, for soup, leave onion whole.
Wash lentils, changing water 3 times.
Place all ingredient except potatos and yoghurt into a 6 quart pot. For soup, add 2 quartes of water and during the cooking, add additional water as needed to keep soup from thickening too much. For stew, use the same, but only add enough additional water during cooking to create a thick product.
Bring to a boil and then reduce to a simmer, adding the potatoes (if used) about 10 minutes after reducing heat, cook until lentils are soft and have burst. (Time varies based on type of lentils used - if you use grey lentis, this may take over an hour - add the potatoes 20 minutes before the hour is up! Red and Orange and Yellow lentils are usually done in half an hour.) If making soup, remove and discard onion.
Before serving, stir in yoghurt, and add a tablespoon of cider vinegar (optional).
Made without the carrots and potatoes, the stew version is terrific with fried potatoes and sauted squash!