To make the filling, peel and chop the apples in small cubes, and add them to a bowl together with the blackberries, cornflour, lemon extract and sugar.
Mix to combine and set aside.
To make the shortcrust pastry, melt the butter and allow it to cool.
Sift the flour in a large bowl, add the melted butter, sugar and lemon extract.
Mix to get a crumbly texture.
Use ⅔ of the mixture to spread over the bottom of a square baking tray (mine is 20x20 cm, or 8x8 inches) - best to either grease the tray, or line it with a non-stick paper.
Press the mixture down well, and bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 15 minutes or until golden.
Spread the fruit mixture over the baked pastry.
Mix the oat with the remaining pastry, then top the fruit with it.
Bake for a further 15 minutes.
Cool down completely, then slice.