Plum and Blueberry Upside-Down Torte

This special recipe comes from Darina Allen of the famed Ballymaloe cooking school in County Cork, Ireland. Darina also likes to use other fruits in season, such as apricots, peaches, or other varieties of plums, but I adore the greengage plums, which are tart and perfumed; the blueberries, when baked, lend their deep purple color to the fruit topping. The torte is firmer and less sweet than most because it has no added liquid: The fruit’s juices moisten it. Making this torte a day ahead al