Wash lentils in cold water and discard bits of rocks. Place in large kettle and cover with 2 quarts of warm water. Bring to a boil (about 5 minutes). Drain and set aside.
Add the olive oil to the pot and saute the onions until translucent, constantly stirring. If you like, you can add celery and carrots. Add the lentils, spices and two quarts of water, and bring to a boil. Cover and simmer until lentils are tender - about a half-hour at a medium to low simmer.
Add the diced potatoes and cook again for half an hour. Serve hot or at room temperature.