Cut room temperature butter into small pieces and add them to the dry ingredients. Use a fork or a pastry cutter to cut the butter into the flour mixture until it resembles coarse crumbs. Set this topping mixture aside for later.
Peel, core, and slice apples into about ¼ inch slices. In another large bowl, combine the apple slices with flour, sugar, cinnamon, and a pinch of salt. Toss the apple slices until they are evenly coated with the sugar and spices.
Transfer the apple mixture into a large cast-iron skillet or deep pie dish, spreading them out evenly. Sprinkle the crumb topping evenly over the apple mixture, making sure to cover the apples completely.
Bake the apple crumble in the preheated oven for about 45 minutes, or until the topping is golden brown and the apple filling is bubbling.
Allow the crumble to cool for a few minutes before serving. Enjoy it warm, perhaps with a scoop of vanilla ice cream or a dollop of whipped cream.