1. Heat the oil in a large skillet over medium heat. Add the onion, carrot, bell pepper, and garlic, cover and cook until softened, about 5 minutes.
2. Transfer the cooked vegetables to a 4-6 quart slow cooker, add the beans, tomatoes and their juice, stock, bay leaves, cumin, thyme, and cayenne, and season with salt and black pepper. Stir to combine. Cover and cook on Low for 8 hours.
3. Remove and discard the bay leaves and taste to adjust the seasonings. Just before serving, stir in the lemon juice, if using. To thicken, purée at least 2 cups or up to one half of the soup solids with an immersion blender used right in the cooker, or ladled into a regular blender or food processor and returned to the cooker. Serve hot.