Spread the lentils on a countertop and remove any pebbles or debris that may be mixed in with them.
In a large saucepan, add the olive oil and onions, and sauté over medium-high heat until the onions begin to change color.
Add the carrots, celery stems, and potatoes, and stir for a minute. Add the split lentils (red lentils) and stir for another minute. Add the water and cumin powder.
Once the mixture starts boiling, reduce the heat, cover, and let it simmer over low heat for 25 minutes.
Blend the mixture using an immersion blender or a regular blender.
Resume cooking over low heat for 5 minutes. If the soup is too thick, add a little more water.
Add some freshly squeezed lemon juice, a drizzle of olive oil, and some freshly ground black pepper upon serving, if desired.