Heat oil in a large soup pot over medium-high heat. Add in the onions, carrots and celery and cook until they become tender, about 6-7 minutes, stirring frequently. Season with salt and pepper to taste.
Stir in the garlic, cumin and cinnamon. Heat until fragrant, about 30-60 seconds. Add lentils to the pan and heat 1-2 minutes to slightly toast. Pour in the chicken broth, water and lemon juice then bring pot to a boil. Reduce heat to low and simmer until lentils are tender, about 30-45 minutes.
Right before serving, stir in the spinach and cook until bright green and wilted, about 2 minutes. Season with additional, salt, pepper and lemon juice to taste before serving warm.