1. Preheat your oven to 375 F (190 C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the bars out later.

2. In a large bowl, cream together the softened butter and 1 cup of granulated sugar until light and fluffy. Beat in the vanilla extract and sourdough discard until well combined. In a separate medium bowl, whisk together 2 cups of all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined and a soft dough forms. Avoid overmixing.

3. Press about two-thirds of the dough evenly into the bottom of the prepared 9×13 inch baking pan. You can use your hands or the back of a spoon to create a firm, even layer.

4. In a medium bowl, combine the diced apples, brown sugar, cinnamon, nutmeg, and lemon juice. Toss gently until the apples are evenly coated with the spice mixture.

5. Spread the prepared apple filling evenly over the crust in the baking pan. Try to get an even layer so each bar has a good amount of apple.

6. Take the remaining one-third of the dough and crumble it over the apple layer. In a separate small bowl, combine the old-fashioned rolled oats, 1/2 cup all-purpose flour, and 1/4 cup granulated sugar. Cut in the cold butter pieces using your fingertips or a pastry blender until the mixture resembles coarse crumbs. Sprinkle this crumble mixture evenly over the top of the apples.

7. Place the baking pan in the preheated oven and bake for 40-45 minutes, or until the topping is golden brown and the apple filling is bubbly and tender when pierced with a fork. The edges of the crust should also be lightly golden.

8. Remove the pan from the oven and place it on a wire rack to cool completely. Allow the bars to cool for at least 30-60 minutes (or even longer) to ensure they set properly, making them much easier to cut into clean, neat squares.