Steps

1. Melt the butter in a nonreactive pot, and gently cook the scallions or leeks in the butter until they are tender.
2. Add the stock. When it has nearly begun to boil, add the potato. Cover the pot, and cook the mixture until the potato pieces are tender.
3. Stir in the sorrel leaves. As soon as they are wilted, whirl the mixture until smooth in a blender. Check for any strings (from the sorrel stems); if you find them, you should strain the soup.
4. Stir in the cream and some salt. If needed, reheat the soup before serving.