In a wide skillet, heat oil over medium and sauté onions, green onions, celery and potatoes until soft. Do not brown! Add vegetable stock and stir well then bring to a boil. Reduce heat and simmer for 15 minutes, until potatoes are soft. Add spinach and stir until leaves wilt, about 30 seconds. Remove from heat immediately or you will lose that beautiful green color! Cool the soup a little and pureé in batches in the blender or food processor. Add a dollop of sour cream and season with salt and pepper.