Preheat oven: Prepare a wide-based, shallow ovenproof dish with a 2-litre (31⁄2 pint) capacity and preheat the oven to 200C/180C fan/gas 6.
Plum filling: Place the plum halves and granulated sugar into the dish and give them a gentle toss.
Crumble topping: Combine the flour and butter in a mixing bowl until it resembles breadcrumbs by rubbing the flour and butter together with your fingertips or otherwise use a food processor. Stir in the demerara sugar and then evenly sprinkle this mixture over the plums, making sure to level the top.
Bake: To prevent any spills in the oven, place the dish on a baking sheet and bake for approximately 30-40 minutes. You'll know it's done when the top turns a pale golden brown and it's bubbling around the edges.
Serve: Serve your delightful dessert with custard, cream, or crème fraîche
Notes
Prepare Ahead. Mary Berry's plum crumble can be prepared up to a day in advance and stored in the refrigerator.
Freezing. The crumble freezes perfectly when assembled but not yet cooked.
For a variation, add ground ginger to the crumble topping.