Rinse the lentils in a sieve until the water runs mostly clear.
Soften the onion and carrots in the oil over medium heat, about 6 minutes, until the onion turns translucent.
Add the garlic and cumin and stir 30 seconds, until the cumin smells toasted.
Add the lentils, broth and salt. Bring to a boil, then drop to a simmer.
Simmer uncovered 25 minutes, stirring now and then, until the lentils have collapsed into a rough, thick puree. Skim any foam in the first minutes.
Off the heat, squeeze in the whole lemon's juice, taste, and adjust salt. Blend a cup and stir it back for a silkier pot, or leave it rustic.