Place the flour, sugar and butter in a large mixing bowl and rub together with your fingers until the mixture resembles breadcrumbs.
In a baking dish arrange the plums in a single layer, cut-side up.
Sprinkle with 1 tablespoon of sugar and ½ teaspoon of cinnamon powder.
Sprinkle the crumble mix evenly over the plums, taking care not to press it down.
Bake for 40 minutes until the crumb is golden brown, the plums are tender and bubbling.
Notes
Serve warm or at room temperature.The plum crumble is perfect itself but you can eat with ice cream, crème fraîche, mascarpone, yogurt, or whipped cream.