Preheat the oven to 190°C, 375°F, Gas 5.
Place the plums in a baking dish, add half the sugar, the orange zest and juice, cinnamon and nutmeg and toss to mix. Bake in the oven for 5 minutes.
Meanwhile, make the topping. Put the flour and salt into a mixing bowl, add the chilled butter and rub it in using your fingertips until the mixture resembles fine breadcrumbs. Stir in the remaining sugar.
Scatter evenly over the plums then return to the oven for 25-30 minutes, until golden brown. Serve warm with single cream or custard.