Heat a large pot over medium heat and add a splash of olive oil. Sauté onions, garlic, carrots, and celery until softened (about 5 minutes).
Stir in lentils and diced tomatoes, coating them in the aromatics before adding vegetable broth.
Add cumin, thyme, and bay leaves; stir well.
Bring to a gentle boil, then reduce heat to low and cover loosely. Simmer for about 30 minutes until lentils are tender.
When done cooking, stir in spinach or kale until wilted (about 2 minutes).
Serve hot with bread or garlic toast.
