Preheat the oven to 180°C (350°F).
In a large bowl, toss the plums with the granulated sugar, cinnamon, and nutmeg. Transfer the mixture to a baking dish.
In a separate bowl, combine the flour, butter, ground almonds, and demerara sugar. Use your fingers to rub the butter into the dry ingredients until the mixture resembles coarse crumbs.
Sprinkle the crumble mixture evenly over the plums.
Bake for 35-40 minutes, or until the topping is golden brown and the plums are tender.
Serve warm with ice cream or whipped cream.