Heat the oil over medium heat. Saute the diced onion until translucent.
Add the salt, pepper, turmeric, and cumin. Cook until fragrant, 30 seconds-1 minute.
Add the washed lentils and water. Bring to a boil, then reduce to the lowest possible temperature. Cook for a minimum of 8 hours. Monitor the water level and add more, if necessary.
Smash some of the lentils against the side of the pot with a wooden spoon to thicken the soup. Serve warm.
Notes
You can cook the lentils in a slow-cooker on the low setting. Monitor the water level to make sure the lentils aren't dry.Often served with fried onions, fresh herbs and lime, and Persian Tea Eggs.