Slowly bring the lilac berries to the boil with 1/8 l water. Stir in the sugar and cook for about 10 minutes, then pass through a sieve into a second saucepan, squeeze the fruit in the sieve well with a spoon. Heat the juice. Peel the apples and cut them into thin wedges, then add them to the juice and let them steep. Add the plums to the soup a few minutes after the apple pieces and let them steep. Season the soup to taste and, if desired, add a little more sugar.
For the dumplings, bring the milk with the butter and a little salt to the boil, stir in the flour until a dumpling forms, allow to cool a little and then stir in the egg. Cut small dumplings from the dough with two teaspoons and let them steep in the soup.