Peel, core, and dice the apples into small chunks, then toss them with lemon juice in a bowl. This keeps them from browning and adds a bright flavor.
In a large mixing bowl, combine the oats, brown sugar, cinnamon, and flour. Mix well to evenly distribute the ingredients.
Add the cold butter pieces to the dry mixture. Use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs, with some larger clumps for texture.
Spread the diced apples evenly in the prepared baking pan, creating a layer that covers the bottom.
Crumble the oat mixture over the apples, pressing it down lightly to ensure it sticks and forms an even topping.
Bake in a preheated oven at 180°C (350°F) for 35-40 minutes, until the topping turns golden brown and the apples are bubbling around the edges, filling your kitchen with warm cinnamon and baked apple aroma.
Remove the bars from the oven and let them cool in the pan for about 15 minutes, allowing the filling to set and making them easier to cut.
Use a sharp knife to cut into squares or bars, wiping the knife clean between slices for neat edges. Serve warm or at room temperature, perhaps with a dollop of whipped cream or a scoop of vanilla ice cream for extra indulgence.
