Warm the oil in a large, wide bottomed pot over medium heat for about 2 minutes. Add the onion, bell pepper, diced tomatoes, garlic, beans, cumin, chili pepper, and salt. Stir and let cook for about 5 minutes to soften slightly. Meanwhile, warm the stock in the microwave for about 1 minute. Add stock to the rest of the ingredients, turn the heat to high to bring just to a boil, then lower heat to medium-low. Let simmer for about 20-25 minutes, or until everything is soft.

Stir in the salsa and puree with an immersion blender. (Alternatively, you can puree in batches in a regular blender, taking precautions when working with a hot liquid.) Taste and add more salt if needed. Serve with shredded cheese, a squeeze of lime, and tortilla strips, as desired.