Photograph: Jonathan Gregson
Preheat the oven to 180°C, fan 160°C, gas 4 and place the plums, cut side up, in a small roasting tin. Sprinkle with the sugar, add the lemon zest, vanilla pod and a splash of cold water. Bake for 15 minutes, then remove and turn over the plums. Scatter the blackberries over the top.
Bake for a further 15 minutes until the plums and blackberries have softened nicely. Remove the fruit from the tin and allow to cool. Taste it (you may need to add a little lemon juice to lift the flavours), remove the lemon zest and vanilla pod. Serve at room temperature with a dollop of Greek yogurt, sprinkled with the crushed amaretti biscuits.
Get ahead: cook the day before, then chill. Bring to room temperature before serving.