Preheat the oven to 375°F (190°C). Position the oven rack in the center of the oven for even baking.

Prepare your pan: Line a 9x13-inch baking pan with parchment paper, leaving a slight overhang on the sides if you want to easily lift the bars out later. Lightly grease the pan or parchment with cooking spray or butter to prevent sticking.

Mix the dry ingredients: In the bowl of a stand mixer fitted with the paddle attachment, combine the gluten-free flour, cane sugar, and salt. Mix briefly to combine.

Cut in the butter: Add the cubed, softened butter. Mix on low to medium speed until the mixture resembles coarse crumbs and starts to clump together. The crust mixture should pull away from the sides of the bowl. The texture should be sandy but hold when pinched.

Reserve for the crumble: Remove 2/3 cup of the mixture and set aside for the topping.

Form the crust: Press the remaining dough in an even layer into the bottom of the prepared baking pan, using the palms of your hands or the bottom of a flat measuring cup to press it down firmly and evenly.

Bake the base: Bake for about 15 minutes, or until the edges are lightly golden brown. Rotate the pan halfway through baking for even browning.

Prepare the apples: Peel, core, and chop about 5–6 medium apples (enough for 6 cups). Aim for ½-inch cubes so the apples cook evenly without turning mushy.

Flavor the filling: In a large bowl, toss the chopped apple slices with cinnamon, lemon juice, and vanilla until well coated. If your apples are extra tart, add 1–2 tablespoons of maple syrup or honey for sweetness.

Combine the ingredients: In a medium bowl, add the brown sugar, pecans, rolled oats, and the reserved cookie base mixture.

Mix until crumbly: Use a pastry cutter, fork, or your fingers to work the mixture together until it forms pea-sized clumps. The mixture should be crumbly, not dough-like.

Layer the filling: Once the crust has partially baked, remove it from the oven. Spread the cinnamon-apple mixture evenly over the warm crust in a single layer.

Add the crumble: Sprinkle the crumble mixture evenly over the apples. Do not press it down—leaving it loose will create a crispier topping.

Bake again: Return the pan to the oven and bake for 20 minutes. Then, loosely cover the pan with aluminum foil to prevent over-browning, and bake for an additional 10 minutes.

Finish baking: Turn the oven off and let the bars rest inside for another 5 minutes. This helps the apples finish softening without overcooking the crust.

Cool or serve warm: Remove from the oven and let the bars cool completely in the pan for neat slices. For a cozy dessert, cut while warm and serve with a scoop of vanilla ice cream or a dollop of whipped cream.