Preheat oven to 180c.
Combine water and lemon juice in a small saucepan, then add sliced apples (see my notes below), white sugar, ground cinnamon, and vanilla essence then mix to combine.
3 green apples - I used Granny Smith, 55 g granulated white sugar, 1 tablespoon lemon juice, ¼ teaspoon ground cinnamon, 1 teaspoon vanilla essence, ½ cup water
Bring to a simmer and cook for 5 - 10 minutes or until apple is just softened.
Meanwhile, make the crumble mixture by adding the dry crumble topping ingredients to a bowl and mixing to combine.
100 g plain flour, 60 g rolled oats, 70 g light brown sugar
Add melted butter and mix to combine (the mixture will be a bit clumpy) then set aside.
80 g salted butter
When apples are soft, drain off the excess liquid then mix in blueberries.
250 g blueberries
Transfer fruit mixture into a baking dish (see my notes for size) then sprinkle the crumble mixture in an even layer over the fruit and bake for 30 minutes or until the crumble is crunchy and golden.