Chop the onion, potatoes, and carrot into small pieces.

In a pot over medium heat, add a little butter and sauté the chopped vegetables for a few minutes. Add the lentils and stir briefly with the vegetables.

Add the water and cook for about 30 minutes, until the vegetables and lentils are soft. Season with cumin and salt.

Blend the mixture in a blender until the soup is smooth. If the soup is too thick, add hot water as needed.