Add the onions, carrots, and celery, and sauté until the onions are translucent, about 5-7 minutes.
Stir in garlic, cumin, and coriander, and cook for another minute until fragrant.
Add the lentils and vegetable broth to the pot. Bring to a boil, then reduce the heat to low and simmer for 30-35 minutes, or until the lentils are tender.
Season with salt and black pepper to taste before serving.