Preheat your oven to 375°F (190°C) and butter a baking dish to prevent sticking.
Place the sliced apples into a mixing bowl, add lemon juice, sugar, and cinnamon, then toss to coat the fruit evenly.
Transfer the coated apples into the prepared baking dish, spreading them out into an even layer.
In a separate bowl, combine oats, flour, and brown sugar, then add the cold, cubed butter.
Use a pastry cutter or fork to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
Spoon the crumble mixture evenly over the apples, covering them completely.
Bake in the preheated oven for about 35 minutes, or until the top is golden brown and bubbling around the edges.
Remove the crumble from the oven and let it cool slightly, then serve warm topped with a scoop of vanilla ice cream that melts into the hot fruit and crunchy topping.