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Chop the 5 tablespoons (73.9ml) of butter into small cubes, keeping them cool until use. Mix the flour, sugar, brown sugar, oats, cinnamon and salt in a large mixing bowl. Cut the butter into the dry ingredients until large clumps form. Keep the mixture cold until it is ready to cook.
Make sure your butter is firm and cold before you start using it. This will give the oats, sugars, and flour something solid to bind onto while they cook, leading to big, delicious crumble topping. It can help to cut the butter into 1/2" cubes ahead of time and place in the freezer while you measure everything else out. The amount of butter in your recipe is up to you. If you want a very rich topping, you can add as many as 89 tablespoons (120130ml), though you should add 12 tablespoons (14.829.6ml) more flour and brown sugar if you do. Use a whisk to lightly mix the dry ingredients, getting rid of any clumps and making sure everything is evenly distributed. Cutting in the butter means you physically mash and cut the butter when surrounded by the flour mixture. This allows the flour to coat as much of the butter as possible, which leads to a crispy, delicious topping. You can cut the butter three ways: Food Processor: Pulse the mixture until there are still 1/4" clumps and none of the butter is uncoated with flour. Pastry Blender: These D-shaped tools have 4-5 wires at the end which effectively cut the butter up into thin slices. Cut until you have a gravel-like consistency. Fingers: Freeze the whole mixture for 2-3 minutes before working, then quickly mash the butter into the flour, aiming for a crumbly, gravel-like mixture. Remember that the colder the butter is when it hits the oven, the better your topping will be. Set the topping in a cool place, like the refrigerator, while you prepare the apples.