Finely chop the onion, carrot, celery, and garlic.
Heat olive oil in a pot and sauté the prepared vegetables.
Add the lentils and sauté together.
Pour in water or vegetable broth.
Add tomatoes, salt, and pepper.
Once it starts boiling, simmer on medium heat for 20-30 minutes until the lentils are tender.
Add lemon juice or herbs to taste.