Preheat the oven to 190°C.
Place the apples and blackberries in a large mixing bowl. Sprinkle over the sweetener, cornflour and mixed spice and mix well. Transfer to an ovenproof dish and spread the fruit out evenly.
Place the flour, sweetener, salt and reduced-fat spread in a mixing bowl and, using your fingers, rub together until it resembles fine breadcrumbs. Stir in the oats, then add the eggs and mix until combined.
Use a tablespoon to place spoonfuls of the mixture onto the fruit, leaving gaps for the fruit to bubble through.
Place in the preheated oven for 25 minutes, until the top is lightly golden. Remove from the oven and cover the top loosely with a sheet of kitchen foil. Return to the oven for a further 20 minutes, until the apples are tender, the fruit is bubbling and the top is golden brown. Serve warm, alone or with custard.