Combine rhubarb, strawberries, and water in soup kettle and simmer, partially covered, 20 minutes. Strain, reserving fruits, and return liquid to kettle. Reheat to simmer. Combine cornstarch and water or wine to form a smooth paste. Add ½ cup hot soup to paste, stir with a whisk until blended, and add mixture to kettle. Cook over low heat, stirring constantly, until smooth. Add sugar and reserved fruits and stir. Combine cream and egg yolk and stir slowly into soup. Do not boil. Cool, chill, and serve cold, garnished with additional sliced berries. (Also good hot.)