Preheat the oven to 180°C (350°F) and brush an ovenproof dish with melted butter.
Add the oil, butter, and sugar to a bowl of an electric mixer fitted with a paddle attachment. Beat at medium-high speed until light in color and well creamed—about 2 to 3 minutes.
With the motor of the mixer running add the egg, and beat until well combined. Scrape down the sides of the bowl and mix again.
Sift together the flour, baking powder, and salt. Add this to the egg and sugar mixture at once and beat at low speed until the dough is just combined. Add the oats and mix to combine.
Wrap the dough in plastic wrap and place it into the fridge to firm up until needed.
Apple Filling
Peel and core the apples, then slice them thinly. Add the sliced apples to a large mixing bowl with the berries, zest, lemon juice, brown sugar, and corn flour. Mix until the fruit is coated with the other ingredients.
Pour the fruit filling into the buttered oven dish and press the fruit mixture down so that it is in an even layer. Grate the chilled pastry over the apple mixture and bake in a preheated oven for about 40 minutes or until golden brown. Serve warm with ice cream or custard.