Heat olive oil in a large pot over medium heat. Add chopped onions, carrots, and celery; cook until softened, about 5 minutes. Add minced garlic and cumin; sauté for another minute. Stir in rinsed lentils, diced tomatoes, and vegetable broth. Bring to a boil, then reduce heat to low and simmer uncovered for 30-35 minutes, or until lentils are tender. Season with salt and pepper. Serve hot, garnished with fresh parsley if desired.