Wash and chop the vegetables: dice the onion, carrots, and celery, and mince the garlic.
In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for 5 minutes until translucent.
Stir in the minced garlic, carrots, and celery. Cook for another 5 minutes until tender.
Add the rinsed lentils, vegetable broth, and diced tomatoes. Bring to a boil.
Reduce heat to low, cover the pot, and let it simmer for 25-30 minutes until lentils are tender.
Add chopped kale or spinach and cook for an additional 5 minutes until wilted. Season with salt and stir in lemon juice.