Rinse the lentils under cold water and set aside.
In a large pot, heat olive oil over medium heat.
Add chopped onions, carrots, and celery; sauté until softened (about 5-7 minutes).
Stir in minced garlic and cook for an additional minute.
Add the rinsed lentils, diced tomatoes, and vegetable broth.
Season with cumin, thyme, salt, and pepper.
Bring to a boil, then reduce heat and simmer for about 25-30 minutes.
Taste and adjust seasoning as necessary.
Serve hot, garnished with fresh herbs if desired.