In a medium saucepan, combine cherries and juice or water. Add sugar, cinnamon and lemon zest. Bring to a boil then simmer gently until cherries are just tender. Dissolve the cornstarch in a small amount of cold water, add to the soup, cook and stir until thickened. Allow soup to cool slightly. Cover and chill the soup for two hours in the refrigerator. Garnish each serving with sour cream or yogurt.