Preheat your oven to 350°F (175°C). In a mixing bowl, combine cold butter, granulated sugar, and all-purpose flour. Blend until the mixture resembles coarse crumbs. Line an 8x8-inch baking pan with parchment paper and press the dough into the bottom of the pan. Bake for 15-20 minutes until lightly golden and set.

Peel and slice about 4 cups of Granny Smith apples. In a skillet over medium heat, combine the apples with granulated sugar and ground cinnamon. Cook for about 5-7 minutes until the apples are softened but still hold their shape. Set aside to cool slightly.

Spread the spiced apple filling evenly over the warm crust, ensuring the apples are uniformly distributed.

In a bowl, mix old-fashioned oats, all-purpose flour, chopped pecans or walnuts, brown sugar, and ground cinnamon. Drizzle in melted butter and mix until the texture resembles wet sand. Spread the topping evenly over the apples.

Bake for 25-30 minutes until the topping is golden brown and crisp. Let cool for about 30 minutes.

Lift the bars out using the parchment paper, slice into squares, and serve warm or at room temperature.