Lightly grease a 10-inch, 9 x 9, 8 x 10 or 11 x 7 inch baking dish. Preheat the oven to 350 F (177 C).
In a large bowl whisk the flour, oats, light brown sugar, ground cinnamon and salt together until well combined. Using a fork, gently mix in a tablespoon of butter at a time until the crumble topping resembles wet sand with some larger crumbs. Transfer the crumble to the fridge until it’s time to top the apple and blueberry filling.
Peel, core and slice the apples into ¼ inch thick slices or ½ inch cubes. Wash and pat dry the blueberries. In another large bowl toss the diced or sliced apples and blueberries with the light brown sugar cornstarch, lemon juice, vanilla extract, salt and cinnamon. Make sure all the fruit is well coated.
Transfer the blueberry and apple mixture to the prepared baking dish. Top the crisp with the brown sugar crumble. Bake the apple blueberry crisp for 45 - 60 minutes or until the top is a beautiful golden brown and the apple and blueberry filling is bubbling around the edges of the baking dish.
Cool the apple berry dessert for 20 - 30 minutes. The filling needs time to cool and thicken. After the crumble has cooled, scoop into a bowl, top with ice cream or whipped cream and enjoy!
