• Preheat your oven to 180°C (355°F). Line an 8-inch (20 cm) square pan with parchment paper, with some paper overhanging.

Make the pastry:

• Add the flour, butter and sugar to a food processor or medium-sized bowl. Pulse the ingredients or cut with a pastry cutter until just combined. Slowly drizzle the cold water into the flour mixture and mix until the pastry comes together. You probably won't need all of the water. If you pinch the pastry between two fingers, it should stick together. However, the pastry shouldn't feel sticky.
• Add half of the pastry to your baking pan. Press into the bottom of the pan to form an even layer.
• Bake the crust for 15-17 minutes or until it's slightly golden brown. Set aside to cool.
• While the crust bakes, add the rolled oats and cinnamon to the dough in the food processor/bowl. Pulse or mix until just combined.

Prepare the apple filling:

• Cut the apples into thin slices. Add the apple slices and rest of the ingredients to a large bowl and toss until combined. Set aside.

Assemble the apple pie bars:

• Layer the apples on top of the parbaked crust. Try to ensure the apples are closely packed together. If there is any excess juice in the bowl, pour it on top of the apples. Sprinkle the crumble on top of the apples.
• Bake the apple pie bars for about 43-48 minutes or until the surface is evenly golden brown. If you insert a toothpick into the center, the apples should feel tender.
• Allow the bars to cool completely in the pan.
• Use a serrated knife to cut the bar into squares. Enjoy the bars as they are. For an indulgent dessert, serve the bars with a scoop of vanilla ice cream and caramel sauce.
• Store leftover bars in an airtight container at room temperature for 1 day, in the fridge for up to 4 days, or in the freezer for up to 1 month. The crumble will soften over time.