Heat the olive oil in a large pot over medium heat. Add the onion, carrots, and celery and cook until softened, about 5-7 minutes. Add the garlic and cook for another minute until fragrant.
Stir in the lentils, vegetable broth, turmeric, cumin, red pepper flakes (if using), and bay leaf. Bring to a boil, then reduce heat and simmer for 30-40 minutes, or until the lentils are tender.
Remove the bay leaf. Stir in the lemon juice and season with salt and pepper to taste. Serve hot, garnished with fresh parsley.
Notes
For a thicker soup, use an immersion blender to partially blend the soup before adding the lemon juice.