Rinse the lentils in a strainer under cold water until the water runs clear.
Heat the oil in a large soup pot set 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.
Stir in the garlic, cumin, and cinnamon. Heat until fragrant, about 60 seconds. Add the lentils to the pan and heat for 1-2 minutes to slightly toast.
Pour in the vegetable broth and lemon juice, then bring the pot to a boil. Reduce heat to low and simmer until the lentils are tender, about 30-45 minutes.
Stir in the greens, season with additional salt, pepper, and lemon juice to taste before serving.