Heat oil in a heavy stock pot over medium-high heat. Add onions and saute until soft, about 5 minutes. Add garlic and saute until garlic begins to brown, 2 to 3 minutes. Stir vegetable broth, sweet potatoes, ginger, cilantro, cumin, cayenne pepper, salt, and ground black pepper into onion mixture; bring to a boil. Reduce heat and simmer until potatoes are soft, about 30 minutes.

Remove stock pot from heat and blend soup with an immersion blender until smooth. Season with salt and ground black pepper as necessary. Serve with a dollop of sour cream.