Peel, core and slice the apples, then toss with lemon juice.
In a large bowl, combine rolled oats, flour, 3/4 cup sugar, cinnamon and baking powder.
Add the 2 tablespoons cold butter and the 1/2 cup cold cubed butter to the dry ingredients.
Rub the butter into the mixture until it resembles coarse crumbs.
Grease a baking dish and sprinkle 3–4 tablespoons of the crumble mixture over the base.
Spread one‑third of the apple slices over the base, then sprinkle more crumble on top.
Repeat the layering twice more, finishing with a generous layer of crumble.
Sprinkle the remaining 1 tablespoon sugar over the surface.
Bake at 180°C / 360°F for 35–40 minutes, or until the top is golden and the apples are tender.
Allow to cool slightly before serving, ideally warm with ice cream or whipped cream.