Heat olive oil in a large pot over medium heat. Add onions and cook until translucent, about 5 minutes.
Stir in garlic, carrots, and celery. Cook for another 3 minutes.
Add cumin and mix until aromatic, about 30 seconds.
Pour in vegetable broth, diced tomatoes, lentils, and bay leaf. Bring to a boil.
Reduce heat to low and simmer, covered, for 25 minutes.
Stir in quinoa, salt, and pepper. Simmer, covered, for an additional 15 minutes or until quinoa and lentils are tender.
Remove bay leaf before serving. Adjust seasoning to taste.