Preheat your oven to 180°C (350°F) and line your baking pan with parchment paper or lightly grease it.
Peel, core, and thinly slice the apples, then toss them with lemon juice to keep their color bright and prevent browning.
In a mixing bowl, combine the oats, flour, brown sugar, cinnamon, and a pinch of salt. Mix until evenly distributed.
Add the cold, small pieces of butter to the dry mixture, then cut in with a pastry cutter or your fingers until it resembles coarse crumbs.
Press half of the crumble mixture evenly into the bottom of the prepared baking pan to form a compact base.
Spread the sliced apples evenly over the crust layer, creating a uniform filling.
Sprinkle the remaining crumble mixture evenly over the apples, covering them completely.
Bake in the preheated oven for 35-40 minutes, until the crumble is golden brown and the filling is bubbly around the edges.
Remove from oven and let cool in the pan for at least 15 minutes to allow the bars to set.
Use a serrated knife to cut into squares or rectangles. Serve slightly warm or at room temperature, perhaps with ice cream or caramel if desired.
