Wash plums, halve, stone and cut them into large pieces. Peel and quarter the apple and cut out the core. Cut apple into small pieces. Mix plums, apple, cinnamon stick, approx. 4 tbsp. water and 3 tbsp. brown sugar in a pot, bring to the boil and simmer for approx. 5 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Melt butter. Mix oat flakes, flour and 100 g sugar. Add liquid butter. Knead everything into crumbles with your hands.
Sort the blackberries, wash and drain them if necessary. Remove the compote from the heat, stir in the berries and fill the compote into an ovenproof dish. Remove the cinnamon stick.
Spread the crumbled dough over the fruit. Bake in a hot oven for about 25 minutes. Remove and serve warm. Whipped cream tastes good with it.