Start this recipe by preparing the filling. Heat the butter in a pan and cook the diced apple for 3-4 minute to soften. Add in the frozen mixed fruits and custard and mix together well. Put aside. To prepare the crumble, place the flour and butter into a large bowl and rub together to form a crumble. Mix in the Scottish porridge oats, sugar, cinnamon and almonds. Grease an oven dish with a little butter and spoon in half of the oat crumble. Spoon over the apple and berry filling and finish off with the remaining oat crumble. Bake in the oven at 200`c for 20-25 minutes, or until light golden brown. Serve the crumble warm.