In a large stock pot, heat up the olive oil and add in the chopped onion, then reduce the heat to low. Sauté for about 5 minutes, or until onion turns translucent.
Add the garlic and spices, and continue to cook for 1 minute.
Add in the sweet potato, vegetable stock and water. Bring a boil then reduce heat. Add 2 cups of cooked black beans and cover with a lid and let simmer for about 25 minutes, or until the sweet potato has softened.
Meanwhile puree the other 2 cups of black beans in a food processor (add water to smooth it out, if necessary) then stir the puree into the soup.
Serve topped with diced avocado, plain Greek yogurt, a squeeze of lime juice, and/or cilantro.