Wash plums, halve, stone and cut them into large pieces. Peel apple, quarter it, remove core. Cut apple into small pieces. Mix apple, plums, cinnamon stick, 4 tbsp. water and 3 tbsp. brown sugar, bring to the boil in a pot and simmer for about 5 minutes
Sort the blackberries, wash and drain them. Knead oat flakes, flour, 100 g brown sugar and butter with your hands to form crumbles. Remove the cinnamon stick from the compote and fill the compote into an ovenproof dish. Spread blackberries on top and sprinkle with crumbles. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 25 minutes. Take out, let cool and serve