Bring the sour cherries to the boil with the sugar and thicken with starch (mix with a little water or cold cherry juice). Let cool down.
In the meantime, bring the milk with the pistachios to the boil. Mix the egg with sugar and starch, pour the boiling milk over it while stirring and put everything back into the saucepan. Bring to the boil over a low flame while stirring constantly, then remove from heat and let cool down.
Whip the whipped cream until stiff and fold into the cold pistachio pudding. Divide the cherry compote between four dessert glasses and pour the pistachio mousse on top.
Dessert should be refrigerated for at least 3 hours before serving. It can be prepared very well the day before.