What you need to do:

Set the oven to 200°C or 180°C for fan assisted. Make the crumble by sifting the flour into a bowl with the salt. Rub in the cold butter until the mixture resembles course breadcrumbs. Stir in the sugar. Prepare the pie dish by greasing it with a knob of butter on a paper towel.

Now make the filling. Peel, core and slice the apples thinly into a deep buttered pie dish, mix with the sugar and lemon juice. Layer the apple mixture then blueberries, pressing down to flatten. Sprinkle the crumble topping mixture evenly over the apple so that it is well sealed. With a piece of damp kitchen towel, carefully go round the pie rim to wipe off any excess butter so that it will not burn on whilst cooking. Bake for 30 minutes until golden brown.

And another thing...

You will need a deep pie dish that takes the apple and topping easily as the apple swells as it cooks and can push bits of crumble off the top if the dish is overfilled, or the juices may overflow. Serve with hot custard, whipped double cream or crème fraiche.