Turn your oven to 350 degrees now. Let it get very hot first. This ensures even cooking for your dessert.

Wash all of your apples very well. Peel the skins off with your peeler. Remove the core from each apple carefully.

Slice the apples into thin, even pieces. Put them into your large mixing bowl. Add the lemon juice to the fruit.

Sprinkle white sugar over the sliced apples. Add half of your ground cinnamon too. Toss the apples to coat them well.

Grease your baking dish with some butter. Pour the apple mixture into the dish. Spread them out into one flat layer.

Grab your medium bowl for the crust. Mix the flour and the rolled oats. Add the brown sugar and remaining cinnamon.

Cut the cold butter into small cubes. Add the butter cubes to the flour. Use your pastry cutter to mix it.

The mix should look like coarse crumbs. Do not overmix the buttery topping. It should have some small butter lumps.

Sprinkle the topping over the apple layer. Cover all of the fruit pieces well. Use your hands to spread it out.

Place the dish into the hot oven. Bake it for about 45 minutes. The top should look very golden brown.

The apple juice should be bubbling. Take the dish out of the oven. Let it sit for ten minutes.