Preheat your oven to 180°C (350°F) and line your baking pan with parchment paper or spray it with non-stick spray to prevent sticking.
Peel, core, and chop the apples into small, even pieces. Toss them with lemon juice and sugar until well coated. Set aside to allow the flavors to meld and to keep the apples from browning.
In a medium bowl, combine the rolled oats, flour, brown sugar, and cinnamon. Mix briefly to distribute the ingredients evenly.
Add the cold, small pieces of butter to the dry mixture. Use your fingers or a pastry cutter to work the butter into the mixture until it resembles coarse crumbs—some larger flakes are fine; this creates that flaky, crisp topping.
Spread the prepared apple mixture evenly in the lined baking pan, creating a uniform layer.
Sprinkle the crumble topping over the apples, pressing gently to help it adhere and create an even layer on top.
Bake in the preheated oven for about 35-40 minutes, or until the topping is golden brown and the apple filling is bubbling around the edges. The aroma of cinnamon and baked apples will fill your kitchen.
Remove from the oven and allow the bars to cool in the pan for at least 20 minutes. This helps them set and makes slicing easier.
Use a sharp knife to cut the cooled bars into squares. Serve warm or at room temperature, perhaps with a dollop of whipped cream or a scoop of vanilla ice cream.
Enjoy your homemade apple crumble bars, savoring the tender apple filling and crisp, buttery topping in every bite!
