Begin by preheating the oven to 350ºF (175C/155C Fan).
Place the apple slices and blackberries in a large mixing bowl.
8 oz blackberries, 4 medium sized apples
Then add the sugar, cornstarch, lemon juice and vanilla bean seeds (if using) and give this a stir to coat the fruit.
2 tbsp cornstarch, 1 tbsp lemon juice, 1 vanilla pod, 2 tbsp granulated sugar
Pour the fruit mixture into the base of a cast iron skillet or a casserole dish and arrange in a roughly even layer.
In a separate mixing bowl, stir together the brown sugar, oats, flour, cinnamon and salt.
1 cup light brown sugar, 1 cup rolled oats, ½ cup all purpose flour, 1 tsp ground cinnamon, ½ tsp salt
Add the melted butter to the oat mixture and stir until you have a crumbly oat crisp mixture.
½ cup unsalted butter
Sprinkle this generously over the fruit mixture in your baking dish.
Bake in the centre of the preheated oven for about 45 minutes until the top is golden and the fruit juices are bubbling at the edges.
Remove from the oven and serve warm or room temperature with ice cream!