Heat the oil in a large pot over medium heat. Add the onion, carrot and celery and cook for four minutes. Add in the garlic, cinnamon, cumin and paprika and stir until fragrant, about another two minutes.
Add the chipotle, bell peppers, tomatoes, lentils, beans and walnuts. Reduce the heat to medium-low and simmer for about 20-25 minutes until thick and the vegetables are soft.
Stir in the chocolate and allow to melt. Season with salt and pepper, to taste.