Preheat the oven to 180°C (Fan).
In the small mixing bowl, combine the cubed apple and frozen blackberries with 1 tbsp of light brown sugar and 1 tbsp of flour. Set to one side.
In the large mixing bowl, combine the oats, light brown sugar, flour and mixed spice (if using) with a spoon.
Mix in the egg and melted butter with the spoon, then get your hands in the bowl and bring the mixture together until it forms clusters.
Pour two-thirds of the crumbly mixture into the prepared square tin and lightly press down with your fingers to create an even layer.
Spread the apple and blackberry mixture on top, then sprinkle the remaining third of crumble mix over the top, don’t worry if there are gaps, you want to see the fruit coming through.
Place in the oven and bake for 35-40 minutes until golden on top and the apple is cooked through. Leave to cool before portioning into 8 bars and serving.