Preheat the oven to 180C / 350F / 160C fan forced.
FOR THE CRUMBLE
In a medium bowl, combine the oats, flour and brown sugar. Grate in the butter. Use your fingers to mix and press it all together until well combined and clumping (notes). Place in the fridge.
FOR THE APPLE FILLING
Peel the apples, remove the cores then cut roughly into 1.5 cm (1/2 inch) cubes. Place them in a casserole dish (about 6 cup capacity).
Add the white sugar, water, corn flour, cinnamon and nutmeg, then mix well until everything is evenly combined. Level them out and try to flatten it down a little.
Scatter the crumble topping over the top.
Bake for 45-50 minutes until the top is golden and it's bubbling at the sides. Let it sit for 5-10 minutes before serving.
I use a standard Australian 20ml tablespoon (=4 teaspoons worldwide)
For best results, you should always weigh ingredients where a weight is the first measurement given. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
All ovens vary – test for doneness 5-8 minutes before the recipe suggests
The streusel mix should be well dispersed and creating small clumps but don’t take it so far that you have a dough.