01 - Preheat your oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring an overhang for easy lifting.
02 - In a large bowl, combine the rolled oats, all-purpose flour, packed light brown sugar, baking powder, ground cinnamon, and salt. Pour in the melted unsalted butter and mix until the ingredients are crumbly and evenly moistened.
03 - Set aside 3/4 cup of the oat mixture for the topping. Firmly press the remaining oat mixture into the bottom of the prepared baking pan to create an even crust.
04 - In a medium bowl, toss the diced apples with granulated sugar, all-purpose flour, ground cinnamon, ground nutmeg, and lemon juice until thoroughly coated.
05 - Spread the prepared apple filling evenly over the oat crust. Sprinkle the reserved oat mixture uniformly over the apples, pressing down gently.
06 - Bake for 35–40 minutes, or until the topping achieves a golden brown hue and the apples are tender.
07 - Allow the bars to cool completely in the pan on a wire rack. Lift the bars out using the parchment paper overhang, cut them into portions, and serve.