Combine the flour, brown sugar, pumpkin pie spice, oats, baking powder and salt together in a large bowl.
Add the butter and use your hands to mix into a coarse crumble.
Measure out one cup of the mixture and set aside.
Press the remaining mixture firmly into a greased 9 inch x 13 inch baking dish.
Spread the sliced apples evenly over the oatmeal mixture in the baking dish.
Sprinkle the remaining cup of oat mixture, that we set aside earlier, over the apples.
Bake for 45 minutes.
Remove from the oven and allow to cool for at least 30 minutes before slicing into bars.
Cooling Time: I recommend waiting at least 30 minutes, or up to one hour, before slicing the bars. The bars will still taste delicious if you slice them fresh out of the oven, but they will not keep their texture and you'll have more of a crumble, than an actual bar!
Apples: I recommend granny smith apples when baking, but pink lady or honeycrisp are also good options.
Storing Leftovers: these bars will stay good, covered on the counter at room temperature, for up to 5 days.
Gluten-Free: to make these bars gluten-free, simply swap the all purpose flour for a one to one gluten-free flour substitute. I have tested this at home and it works perfectly!