Rinse the lentils thoroughly with water, then drain. Place them in a pot and add about 9 cups of water (about 2.1 liters).

Cut the onions into medium-sized pieces, then add them to the pot with the lentils and water. Place the pot over medium heat.

Stir the lentils occasionally. When the lentils start to boil, reduce the heat, cover the pot, and let it cook for 40 minutes.

Once the lentils and onions are tender, add the salt, cumin, and black pepper.

Remove the pot from the heat. Blend the lentils and onions well with a blender until the soup is thick and smooth.

Return the blended soup to the pot and place it back on the heat. Stir in the vegetable oil and cook for another 5 minutes. Remove from the heat—your lentil soup is ready to serve.