In a large pot over medium heat, sweat onions in 1 Tbsp oil and season with a healthy pinch each salt and pepper (amounts as original recipe is written // use more or less if altering batch size). Stir and continue cooking on medium heat until translucent and soft.
Add sweet potato and any desired spices at this time (I went for chili, cinnamon, cumin, and chipotle powder - all are optional). Cook for 3 minutes. Then add salsa, water, and vegetable stock.
Bring mixture to a low boil on medium high heat and then lower heat to medium-low and simmer. Add black beans, cover and cook for at least 20 minutes more, preferably 30 - or until the sweet potatoes are fork tender and the soup and thickened. This soup is at its best when prepared the night before or allowed to rest for a few hours so the flavor marries with the vegetables and beans.
Serve with fresh cilantro, onion, avocado, and/or lime juice. Chips make an excellent spoon.
*Inspired by GimmeSomeOven - queen of 5-ingredient soups! *Nutrition information is a rough estimate.