In a large pot, heat olive oil over medium heat. Add chopped onion, garlic, carrots, and celery. Cook, stirring occasionally, for about 5 minutes, until softened.
Add the lentils, diced tomatoes, vegetable broth, cumin, coriander, turmeric, bay leaf, salt, and pepper. Stir to combine.
Bring the mixture to a boil, then reduce heat and let simmer uncovered for 30-35 minutes, or until the lentils are tender.
If using spinach or kale, add it to the soup and cook for an additional 5 minutes.
Stir in the lemon juice, if desired, for a fresh burst of flavor.
Serve hot and enjoy your nutritious and comforting Lentil Soup!