Heat olive oil in a large pot over medium heat. Add chopped onion, carrot, and celery; cook, stirring occasionally, until vegetables soften, about 5 minutes.
Add minced garlic, cumin, turmeric, and cinnamon; cook, stirring, for 1 minute until fragrant.
Pour in lentils and vegetable broth. Bring to a boil, then reduce heat to low and simmer, uncovered, until lentils are tender, about 20–25 minutes.
Season with salt and black pepper to taste. Remove from heat and stir in lemon juice.
For a smoother texture, use an immersion blender to partially blend the soup, or leave it chunky per your preference.
Serve hot garnished with fresh parsley if desired.