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.
Notes
We usually use spinach but you can also swap for other greens like Swiss chard or mustard greens. Leftovers taste even better on days 2 and 3!