Chop the onion into small pieces and sauté with a little oil in a pot over medium heat until the onion softens. Add the water and lentils.

Bring the water and lentils to a boil. Cook for about 30 minutes, or until the lentils are tender. Once the lentils are cooked, turn off the heat and let the mixture cool slightly. Blend the soup well in a blender. After blending, return the soup to the pot over the heat. Add a pinch of salt to taste. If you prefer a thinner soup, add a little more water. Let the soup simmer until it reaches your desired consistency, then turn off the heat.

Serve hot. Sprinkle a little cumin on top and add lemon juice to taste.