Cook fruit in water until tender, 20 to 30 minutes.
Puree fruits by pressing through a sieve or using an electric blender.
(Fruit may be pitted raw for easier handling, but the taste will be less subtle.) Stir potato flour into liquid in which fruit was cooked.
Bring to boiling, stirring until thickened.
Pour thickened liquid into fruit puree.
Stir in sugar, lemon peel and juice, if desired, and salt.
Cook, stirring, 3 minutes.
Serve hot or cold.
Spoon sour cream on top of each serving of soup.
Garnish with croutons.