Heat oil in a pot over medium heat. Cook onion and carrots for 6–8 minutes, stirring.
Add garlic and cumin; stir for 30 seconds.
Add lentils, tomatoes, and broth. Bring to a boil, then reduce to a gentle simmer.
Cook uncovered for 18–25 minutes, stirring occasionally, until lentils and carrots are tender. Add water if needed.
Taste and finish with lemon juice and seasoning. Blend part of the soup only if you want a smoother texture.
Storage: Refrigerate perishables promptly in shallow containers. Use refrigerated leftovers within 3–4 days or freeze earlier. Reheat cooked leftovers to 165°F; keep cold meals chilled during transport. Follow ingredient labels if they require a shorter storage time.