Chop the onion and fry it with the oil in a large pot until golden brown. Add the pureed lentils, water, lime juice and cumin. You may add more water as needed. Simmer for 10 minutes.
Transfer to serving bowls, garnish with chopped parsely and serve. The soup will taste even better tomorrow. When reheating you might want to add some water since it tends to thicken in the refrigerator.