Heat the oil in a large pot over medium heat.
Add the onion, carrot and celery and cook for 4 minutes.
Add in the garlic, cinnamon, cumin and paprika and stir until fragrant, about another 2 minutes.
Add the chipotle, bell peppers, tomatoes, 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.