In a large pot, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic and cook for another minute until fragrant.
Stir in the sliced carrots and chopped celery, and cook for about 5 minutes.
Add the rinsed lentils, dried oregano, dried basil, diced tomatoes with their juices, and vegetable broth to the pot.
Bring the mixture to a boil, then reduce the heat and simmer uncovered for 30-35 minutes, or until the lentils are tender.
Season with salt and pepper to taste before serving.