Preheat the oven to 375°F / 190°C (170° fan-assisted).
Peel and core the apples, then cut them into bite-sized pieces.Place them into a bowl.
To the apples, add the sugar, flour, cinnamon, and lemon juice, then mix everything together to coat the apples. Set aside while you prepare the crumble.
Melt the butter in the microwave in 30-second bursts until completely melted.
In a separate bowl, stir together all the dry ingredients for the crumble.
Pour the melted butter into the dry ingredient mixture, then use a fork to mix it through until all the flour is coated in butter, and you have large clumps of crumble.
Tip the apple chunks into a baking dish and spread them out in an even layer.
Use your hands to sprinkle the crumble topping all over the apples - make sure every part of the apples are covered generously in crumble.
Bake for 35-40 minutes until the crumble is crisp and golden brown, and you can see the apple syrup bubbling at the edges.
Remove from the oven and let the crumble sit for 10 minutes before serving - both so you don't burn your tongue, and also to allow the juices to thicken up a little.
Notes
Apples - I find the best apples for apple crumble are Granny Smith apples as they are a little more tart and have the perfect bite. You can, of course, use any apples you like though. It is also possible to use canned apples for this recipe, though they will be a little more tender than fresh apples. Make sure you drain off all excess liquid from the can before using. Dish Size - this recipe makes enough for an 8x8-inch baking dish. If you are using a 9x13-inch dish, double the recipe.