Trim the stem ends of the chard then pull the leaves off the stems. Chop both the leaves and stems and set aside separately.
Saute onions in olive oil until softened. Add the potatoes and stock and bring to a boil, reduce the heat, cover and simmer for 10 minutes or until the potatoes start to become tender. Add the stems from the greens and cook 5 minutes, then add leaves and cook until tender, about 5-10 more minutes. Cool slightly, then puree in a blender or food processor.
Add the sour cream, reheat gently and season to taste.