01 - Preheat oven to 350°F. Grease an 8-inch square baking dish with butter or cooking spray.
02 - In a large bowl, combine apple slices with lemon juice, granulated sugar, and cinnamon. Toss until evenly coated, then spread the apple mixture in the prepared baking dish.
03 - In a separate bowl, whisk together flour, rolled oats, brown sugar, salt, and cinnamon. Add cold butter cubes and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in chopped nuts if using.
04 - Sprinkle the crumb topping evenly over the apple layer, covering completely.
05 - Bake for 35 minutes until the topping is golden brown and the apples are tender when pierced with a fork.
06 - While the treat bakes, whisk together powdered sugar, milk, and vanilla extract in a small bowl until smooth and creamy.
07 - Remove from oven and let cool for 10 minutes. Drizzle the frosting over the warm crumble before serving.