Place the plums in a shallow roasting tin or baking dish. Add the orange rind and juice and sprinkle over 50 g (2 oz) of the sugar and the cinnamon. Dot half the butter on top and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes, spooning over the juices halfway through, until the plums have softened.
Meanwhile, line a large baking tray with foil, then scatter over the oats, hazelnuts and remaining sugar. Dot with the remaining butter and place in the oven with the plums for 5–10 minutes until golden and toasted, stirring once to coat in the melted butter.
Spoon the compote into 4 serving dishes, scatter over the crunchy oat granola and serve with yogurt.