Rinse the lentils and set aside.
In a large pot, sauté the onion, carrots, celery, and garlic in a bit of olive oil until softened, about 5 minutes.
Add the lentils, broth, diced tomatoes, bay leaf, and oregano. Bring to a boil, then reduce heat and simmer for 20-25 minutes, until lentils are tender.
Season with salt and pepper to taste.
Serve hot, garnished with grated Parmesan cheese if desired.
