Heat oil in large saucepan. Add onion and cook until soft, about 2 to 3 minutes. Add stock plus 2 cups water, then add sweet potato, carrot, and ginger. Bring to a boil, reduce heat, and simmer until vegetables are tender, about 15 minutes.
Strain out vegetables and put them in a food processor or blender. Puree until smooth, adding a bit of broth if needed. (If you're using a standard blender, allow the mixture to cool first; hot liquid may cause the blender to squirt out contents. Depending on the size of your blender, you may have to do this in batches.)
Pour vegetable puree back into the saucepan and stir until well blended and smooth. Add salt and pepper to taste. Serve with a dollop of yogurt on top, if desired.